The effect of race on differences in metabolic control was examined in patients with non-insulin-dependent (NIDDM) and insulin-dependent (IDDM) diabetes mellitus. Data were collected on HbA1c, age, duration of diabetes, age at onset, family function, stress, body mass index, waist/hip ratio, total cholesterol, insulin dose, diet, and physical activity. Among those with NIDDM, black patients had significantly higher HbA1c levels than their white counterparts. This difference persisted after adjustment for covariates. Among patients with IDDM, black subjects were found to have higher HbA1c levels, body mass index, and total cholesterol levels than their white counterparts. After correction for diabetes duration, relative insulin dose, physical activity, body mass index, and cholesterol, black women had significantly higher HbA1c levels than black men, white men, or white women. We conclude that race and sex differences do affect the metabolic control of patients with diabetes mellitus.