Serum uric acid has been described as being increased in the prediabetic stage of diabetes mellitus and as being decreased in overt diabetes. In this study we compared the serum uric acid levels of patients with insulin-dependent diabetes mellitus (IDDM) to those of controls matched for sex, age and ethnic origin. Also the correlation between serum uric acid levels and the fractional excretion of uric acid in IDDM patients was investigated, as well as the correlation between glycosuria and the fractional excretion of uric acid. The mean serum uric acid was lower in IDDM patients than in normal controls (4.0 ± 1.3 vs. 4.3 ± 1.3 mg/ 100 ml; p < 0.03), mainly due to significantly lower levels in male and Ashkenazi IDDM patients, as compared to their respective controls. The fractional excretion of uric acid was found to be elevated in IDDM patients: 13.0 ± 8.6% (mean ± SD). A significant negative correlation was found between serum uric acid levels and the fractional excretion of uric acid in IDDM patients (p < 0.001), although not when the males were examined separately. We found no correlation between the fractional excretion of uric acid and the degree of glycosuria in IDDM patients. In addition, the prevalence of hypouricemia (serum uric acid < 2.5 mg%) was the same in IDDM patients and contr