Direct investigation of the polyol pathway is rarely possible in studies of human diabetes. A spectrophotometric assay has been developed for the measurement of aldose reductase and sorbitol dehydrogenase activity in the neutrophil. Neutrophil aldose reductase activity was increased in patients with Type 1 diabetes with complications (median 40 (interquartile range 28-48) u, where 1 unit of enzyme activity = nmol NADPH min-1 10(8)-cells-1) compared with those without complications (20 (16-36) u, p less than 0.01) and normal control subjects (20 (8-36) u, p less than 0.01). In Type 2 diabetes, patients with complications also had higher aldose reductase activity (40 (28-52) u) than those without complications (24 (16-36) u, p less than 0.01). There were no differences between patients without complications and normal control subjects. Sorbitol dehydrogenase activity was decreased in diabetic patients (p less than 0.02) but not significantly different between diabetic patients with and without complications.