Risk factors for serious suicidal behavior among older adults were examined in a case control study of 53 adults aged 55 and older who died by suicide or made medically serious suicide attempts and who were compared with 269 randomly selected comparison subjects. Multivariate analyses suggested that risk of serious suicidal behavior was elevated among those with current mood disorders (OR = 179, CI = 52.8–607.6), psychiatric hospital admission within the previous year (OR = 24.4, CI = 1.9–318.7), limited social network (OR = 4.5, CI = 1.4–14.6). The predominant role of mood disorders was confirmed by population attributable risk (PAR) estimates (73.6%), suggesting that the improved detection, treatment and management of mood disorders should be the primary focus of suicide prevention strategies for older adults.