In patients with the nephrotic syndrome, it is often desirable to assess the disease process, not only by proteinuria but also by indices of glomerular inflammatory process. We investigated the importance of beta-microglobulin (betaM) as a means of assessing renal function in patients with the nephrotic syndrome with normal or abnormal values of creatinine clearance. There were 46 patients (mean age, 42.2 + 10.4 years; male/female (M/F) ratio = 31/15) and 35 healthy controls (mean age 39 + 4.5 years, M/F ratio 25/10). We subdivided the study patients into group A (n = 18, mean age 39.6 + 10.6 years, M/F ratio 8/10) and group B patients (n = 28, mean age 45.6 + 8.9 years, M/F ratio 23/5) who had normal and abnormal values of creatinine clearance respectively. An enzyme-linked-immunosorbent assay (ELISA) was used to quantitate plasma beta2M in the study patients and controls. The median 132M levels of the study patients and controls were 44.0 and 1.7 mg/l respectively (p < 0.0001). Beta-2-M levels correlated significantly with serum creatinine (r = 0.56, p < 0.0001), and creatinine clearance (r = -0.6, p < 0.0001). In group A patients, the median beta2M level was significantly higher than normal (4.1 vs. 1.7 mg/1, p < 0.01). Plasma beta2M levels did not correlate well with any other parameter measured in group A patients. When groups A and B were compared, the median plasma beta2M level in group B was significantly higher than group A (20.3 vs. 4.1 mg/1, p < 0.0001). The urinary beta2M (expressed per mg urine creatinine) was also higher in group B than group A patients (6.8 vs. 0.7 p < 0.05). We conclude that elevation of beta2M-microglobulin in patients with the nephrotic syndrome who have normal creatinine clearance suggests early abnormal renal function in these patients. It may be used to assess the rate of normalisation of renal function or progression to chronic renal failure.