Background. Alcohol abuse among patients with diabetes mellitus is dangerous and complicates therapy, but its prevalence and the factors that predict it are unknown. This study examined the prevalence of problem drinking among a large number of primary care diabetic patients, exploring its relation to age, race, sex, psychological factors, and other health behaviors. Methods. Volunteers with insulin-dependent diabetes mellitus and non-insulin-dependent diabetes mellitus were surveyed at three primary care practice sites. Patients completed a health risk appraisal designed to elicit alcohol use and other health practices, and two psychometric instruments: the Brief Encounter Psychosocial Instrument and the Affect Balance Scale. Fasting blood glucose and hemoglobin Alc levels were also determined. Results. Of 395 diabetic patients, 32 (8.1%) had a drinking problem as defined by answering yes to the question ''Have you ever had a drinking problem?'' or reporting their last drink to be within 24 hours, or both. Patients with a drinking problem coped less well with psychological stress and had a more highly negative affect than those without a drinking problem. Depression, black race, and male sex were significantly associated with problem drinking (odds ratios = 8.42, 2.70, and 3.80, respectively). Problem drinking did not predict glycemic control but was associated with smoking and less frequent glucose monitoring. Conclusions. The prevalence of problem drinking among patients with diabetes mellitus appears lower than among other medical outpatient populations and is in keeping with the prevalence found in community surveys. While the lack of association between problem drinking and glycemic control in diabetic patients may be surprising, these data help define the characteristics of this subgroup of diabetic patients and highlight the need for family physicians to intensify alcohol screening efforts in this population.