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Background Asthma was the most common chronic condition among adults hospitalized for 2009 pandemic influenza A (H1N1) ( pH1N1). Objectives We describe the epidemiology and factors for severe outcomes among adults with asthma who were hospitalized or died from pH1N1 in California. Methods We reviewed California Department of Public Health pH1N1 reports from April 23, 2009 through August 11, 2009. Reports were included if the patient had pH1N1 (or non-subtypeable influenza A) infection by polymerase chain reaction in an adult (age ≥ 18 years) with asthma who was hospitalized or died. Patients were classified as having intermittent or persistent asthma on the basis of regular medications. Risk factors associated with severe outcomes (i.e., intensive care unit admission or death) vs those with less severe outcomes were assessed by chi-square tests and logistic regression. Results Among 744 identified patients, 170 (23%) had asthma (61% intermittent, 39% persistent). 132 of 142 (93%) patients had other chronic medical conditions. Severe outcomes occurred in 54 of 162 (33%), more commonly among those with renal disease (64% versus 31%; P = 0.04) and chest radiograph infiltrates (54% versus 11%; P < 0.01), less commonly among those who received antivirals within 48 hours of symptom onset (22% versus 44%; P = 0.02). In multivariable analysis, chest radiograph infiltrates were associated with severe outcomes (adjusted odds ratio 9·38, 95% confidence interval 3·05-28·90). Conclusions One third of adults with asthma who died or were hospitalized with pH1N1 experienced severe outcomes. Early empiric antiviral therapy should be encouraged, especially among asthma patients. [ABSTRACT FROM AUTHOR]