Forty cases of mesangiocapillary glomerulonephritis are reviewed for whom both light and electron microscopy and full clinical data were available. Immunofluorescence microscopy (IF) was performed on 23 cases and complement screening (CH50, C4 and C3) on 25 cases, with follow-up period of 5-22 y. The results of EM revealed 17 cases (42 per cent.) of Type I and 23 cases (58 per cent.) of Type II MCGN but only 52 per cent. of Type II cases were correctly identified by light microscopy. Epimembranous deposits were seen as frequently in Type II as in Type I (26 per cent. and 30 per cent.) and fragmentation of glomerular capillary basement membranes (GBM) was seen in 27 per cent. of Type I cases. Overall patient survival was 49 per cent. at 10 y and that of patients who presented with nephrotic syndrome was poor (39 per cent. at 10 y). Persistent hypocomplementaemia with C3 Nephritic Factor was present in 40 per cent.; the survival of these patients was less than those with normal complement levels (70 per cent. vs 100 per cent. at 5 y) and they were also more likely to develop renal failure. Renal failure was more likely to develop in those with a creatinine clearance of less than 100 ml/min at presentation and where the biopsy showed substantial crescents in greater than 20 per cent. of glomeruli. Mean CH50, C3 and C4 was lower in the hypocomplementaemic as compared to normocomplementaemic patients, and there were no differences between Type I and Type II. IF showed immunoglobulins and fibrin as well as C3 in both Type I and Type II cases. Our results support the concept of an immune-complex mediated phase in both types of MCGN, and we further suggest that (a) epimembranous deposits are common in both Type I and Type II and (b) cases with fragmentation of the GBM should be designated Type Ia.