While problem drinking is believed to be over-represented in primary care practice, additional research in this area is needed. A probability sample of 394 patients attending all county-operated primary care clinics in Contra Costa County, California, were breathalyzed and interviewed regarding drinking patterns and alcohol problems. These data are compared with those obtained from a representative general population sample of over 3000 respondents living in the same county. While the clinic population reported higher rates of abstinence compared with the general population (38 versus 17%), among drinkers the clinic sample reported higher rates of heavy drinking. In the clinic sample 14% reported a physical health problem related to drinking and 22% reported three or more symptoms of alcohol dependence during the last year, compared with 3 and 10%, respectively, in the general population. The clinic sample was demographically different from those in the general population which could account, in part, for differences in heavy drinking and alcohol-related problems between the two populations. The prevalence of heavy and problem drinking in this primary care practice suggests the potential of primary care settings for early identification and treatment of alcohol-misusing patients.