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Background Cancer risk and Lhermitte-Duclos disease (LDD) risk estimates for Cowden syndrome (CS) are broad and based on a small number of patients. Risk estimates are vital to the development of diagnostic criteria, genetic counseling, and cancer surveillance. To further elaborate and estimate the risks associated with CS, a large cohort of patients was evaluated. Methods CS patients were identified from the medical literature and the Mayo Clinic's records. All patients met accepted diagnostic criteria for CS. Results A total of 211 CS patients (age 44 ± 16 years, 64% female, 46% PTEN mutation) were included (published literature 90% and Mayo Clinic series 10%). The cumulative lifetime (age 70 years) risks were 89% for any cancer diagnosis (95% confidence interval (CI) = 80%,95%), breast cancer [female] 81% (CI = 66%,90%), LDD 32% (CI = 19%,49%), thyroid cancer 21% (CI = 14%,29%), endometrial cancer 19% (CI = 10%,32%), and renal cancer 15% (CI = 6%,32%). A previously unreported increased lifetime risk for colorectal cancer was identified (16%, CI = 8%,24%). Male CS patients had fewer cancers diagnosed than female patients and often had cancers not classically associated with CS. Seven percent of breast and thyroid cancers occurred in patients who were younger than the recommended age to commence radiographic cancer screening. There was a trend for patients with a family history of CS and PTEN mutations to have a lower cancer risk than those without. Conclusions This study confirms CS patients are at increased risk for cancer and quantitative data is provided to guide clinical care. Based on a different tumor spectrum, separate male and female clinical CS diagnostic criteria may be indicated.