Background For community-dwelling older persons with dementia, the presence of multimorbidity can create complex clinical challenges for both individuals and their physicians, and can contribute to poor outcomes. We quantified the associations between level of multimorbidity (chronic disease burden) and risk of hospitalization and risk of emergency department (ED) visit in a home care cohort with dementia and explored the role of continuity of physician care (COC) in modifying these relationships. Methods and findings A retrospective cohort study using linked administrative and clinical data from Ontario, Canada, was conducted among 30,112 long-stay home care clients (mean age 83.0 ± 7.7 y) with dementia in 2012. Multivariable Fine–Gray regression models were used to determine associations between level of multimorbidity and 1-y risk of hospitalization and 1-y risk of ED visit, accounting for multiple competing risks (death and long-term care placement). Interaction terms were used to assess potential effect modification by COC. Multimorbidity was highly prevalent, with 35% (n = 10,568) of the cohort having five or more chronic conditions. In multivariable analyses, risk of hospitalization and risk of ED visit increased monotonically with level of multimorbidity: sub-hazards were 88% greater (sub-hazard ratio [sHR] = 1.88, 95% CI: 1.72–2.05, p < 0.001) and 63% greater (sHR = 1.63; 95% CI: 1.51–1.77, p < 0.001), respectively, among those with five or more conditions, relative to those with dementia alone or with dementia and one other condition. Low (versus high) COC was associated with an increased risk of both hospitalization and ED visit in age- and sex-adjusted analyses only (sHR = 1.11, 95% CI: 1.07–1.16, p < 0.001, for hospitalization; sHR = 1.07, 95% CI: 1.03–1.11, p = 0.001, for ED visit) but did not modify associations between multimorbidity and outcomes (Wald test for interaction, p = 0.566 for hospitalization and p = 0.637 for ED visit). The main limitations of this study include use of fixed (versus time-varying) covariates and focus on all-cause rather than cause-specific hospitalizations and ED visits, which could potentially inform interventions. Conclusions Older adults with dementia and multimorbidity pose a particular challenge for health systems. Findings from this study highlight the need to reshape models of care for this complex population, and to further investigate health system and other factors that may modify patients’ risk of health outcomes., In a retrospective analysis of routinely collected data, Luke Mondor and colleagues examine the associations between multimorbidity and healthcare utilization among home care clients with dementia in Ontario, Canada., Author summary Why was this study done? The co-occurrence of multiple chronic conditions in an individual (multimorbidity) has been linked to poor outcomes including increased hospital use, longer length of stays, and worse cognitive and physical functioning. Particularly for community-residing older adults with dementia, multimorbidity can result in challenges to both self-care and provided care. Individuals with multimorbidity often receive care from multiple physicians across different care settings each year. This lack of physician continuity may lead to poorer quality of care and outcomes. Important gaps exist in our understanding of the interplay between multimorbidity, health system use, and continuity of physician care specifically for individuals with dementia. Our historical cohort study was designed to estimate the risk of acute care hospitalization and emergency department (ED) visit by level of multimorbidity (i.e., chronic disease burden) among persons with dementia in the community. We were especially interested in whether the risk of these health outcomes was lower for those with better continuity of physician care. What did the researchers do and find? We performed a retrospective cohort study of 30,112 home care clients with dementia in Ontario, Canada, using routinely collected health and clinical information linked at the individual level. We defined the level of multimorbidity (i.e., chronic disease burden in addition to dementia diagnosis) based on a count of the presence of 16 common chronic conditions, and compared time, in days, from a health assessment to initial hospitalization (for any cause) and ED visit (not resulting in an inpatient stay) in persons with different multimorbidity levels. We accounted for other possible outcomes including death or placement in a long-term care facility. We found that multimorbidity was highly prevalent in this population—89% of the cohort had been diagnosed with two or more conditions in addition to dementia. In multivariable analyses, we found that the risk of hospitalization and ED visit increased with each higher level of multimorbidity. These associations were comparable in clients with dementia who had high and low degrees of physician continuity. In other words, continuity of physician care did not modify the association between level of multimorbidity and the outcomes. What do these findings mean? Multimorbidity is the norm rather than the exception among older adults with dementia in the home care sector. This increased chronic disease burden is associated with a greater likelihood for costly hospital admissions and emergency visits. With increases in life expectancy, improvements to disease detection, and a shift to community-based care, use of home care services and the prevalence of multimorbidity among older persons with dementia will likely rise. Data from this study may be useful in identifying at-risk individuals and prioritizing the deployment of limited healthcare resources.