Background: Understanding factors that contribute to high suicide risk holds important implications for prevention. We aimed to examine the sociodemographic and medical predictors of attempted suicide (severe enough to require hospitalization) and of completed suicide in a large population-based sample from a health maintenance organization (HMO) in northern California, USA. Method: We designed a cohort study, including 87,257 women and 70,570 men aged 15 through 89 years old at baseline (in 1977-1985) with follow-up for hospitalizations and mortality through the end of 1993. Results: After a median of 10 years, 169 first hospitalizations for attempted suicide (111 among women, 58 among men) and 319 completed suicides (101 among women, 218 among men) were identified. There was a greater incidence of hospitalization for suicide attempt in women than in men and, conversely, a greater incidence of completed suicide in men than in women. The predominant methods of attempted and completed suicides were ingestion of psychotropic agents and use of firearms, respectively. In gender-specific multivariate analysis of hospitalization for suicide attempt, statistically significant associations were seen for age 15-24 years (women), 65-89 years (men), white race (women), 12th grade or less education (both genders), technical/business school education (men), never being married (men), history of emotional problems (both genders), history of family problems (women), history of job problems (men) and presence of one or more comorbidities (men). The independent predictors of completed suicide were: age 15-24 years (both genders), Asian race (women), Caucasian race (both genders), never being married (both genders), being separated/divorced (women), prior inpatient hospitalization for suicide attempt (both genders) and history of emotional problems (both genders). Conclusion: These findings could help health professionals be more effective in the prevention of suicide morbidity and mortality.