N-Acetyl-beta-D-glucosaminidase (NAG) excretion was measured in early morning urine samples from 133 Albustix-negative, normotensive insulin-dependent diabetic patients and 89 non-diabetic controls. Urinary NAG activity was determined using a chromogenic substrate, 2 methoxy-4-(2'-nitrovinyl)-phenyl 2-acetamido-3-deoxy-beta-D-glucopyranoside, and expressed as mumol MNP released/hour/mmol of creatinine. Overall, diabetic patients were found to have a significantly elevated mean urinary NAG activity (p less than 0.01) compared to controls. Within the diabetic patients urinary NAG activity was significantly elevated in patients with either microalbuminuria (p less than 0.001) or "poor" glycaemic control (p less than 0.001), but not in those with retinopathy (p = 0.117). Three-way analysis of variance revealed that the relationship of raised urinary NAG to microalbuminuria and "poor" glycaemic control were statistically independent. Elevated urinary NAG excretion in insulin-dependent diabetes mellitus appears to be associated with early diabetic nephropathy and poor long-term glycaemic control.