We evaluated urinary N-acetyl-beta-glucosaminidase (NAG) excretion in overnight and in second morning urine in 50 young diabetic patients, aged 7.4-25 years with a disease duration from 2-19.6 years. In all patients we evaluated urinary NAG and creatinine excretion, in both overnight and second morning urine, glycosuria, fasting blood glucose and HbA1c levels, insulin requirement, blood pressure, and the presence of microangiopathic complications. Urinary NAG excretion was also evaluated in 69 age- and sex-matched controls. NAG was determined using 3-cresolsulfonphtaleinyl-beta-N-acetylglucosaminide as substrate (Boehringer Mannheim, Germany). In the diabetic patients NAG/Cre ratios were significantly higher than in controls both in overnight and second morning urine (P < 0.0005, respectively). We observed significantly higher NAG/Cre ratio levels in the second morning than in overnight urine, both in controls and in diabetics (P < 0.0005, respectively). Elevated (above 2 S.D. of the mean) NAG/Cre ratios were found in 17/50 patients (34%) in overnight urine and in 29/50 (58%) in second morning urine. No correlation was observed between NAG/Cre ratio levels and age, duration of disease, pubertal stage, body mass index, fasting blood glucose, glycosuria, insulin requirement and blood pressure. The patients with one or more complications did show NAG/Cre ratio levels significantly higher than those without complications (P < 0.005) in second morning urine, but not in overnight urine. Our study has demonstrated an increased rate of urinary NAG excretion in young IDDM patients, in particular in those with microangiopathic complications.