This study measured plasma β<sub>2</sub>-microglobulin (β<sub>2</sub>-m) in patients with mesangial IgA nephritis. Plasma β<sub>2</sub>-m was measured in 51 patients with IgA mesangial nephritis and in 50 normal controls using a Phadebas β<sub>2</sub>-m RIA kit available from Pharmacia Diagnostics (Uppsala, Sweden). The mean plasma β<sub>2</sub>-m in IgA nephritic patients (1.92 ± 0.67 mg/l) was significantly different from that of healthy controls (1.33 ± 0.41 mg/l; p < 0.001). The mean plasma β<sub>2</sub>-m in non-IgA nephritic patients (1.83 ± 0.73 mg/l) was also significantly different (p < 0.001). Patients with IgA nephritis with glomerular sclerosis (n = 33) had significantly higher levels of β<sub>2</sub>-m (2.02 ± 0.70 mg/l) than IgA nephritic patients without glomerular sclerosis (n = 18, 1.72 ± 0.65 mg/l; p < 0.025). In the group with IgA nephritis and glomerulosclerosis, raised β<sub>2</sub>-m levels were correlated with the severity of proteinuria (r = 0.41) (p < 0.02) as well as the intensity of IgA staining on immunofluorescence (r = 0.34; p < 0.05). Elevated β<sub>2</sub>-m levels in IgA nephritis may serve as a useful prognostic marker.