Abdominal complications were detected and investigated in 19 (10%) of 191 children with nephrotic syndrome who experienced 35 episodes of these complications. Fourteen children were Indian with steroid-responsive nephrotic syndrome, and 5 were black, of whom 4 had membranous nephropathy and 1 focal proliferative nephritis. All had clinical features of peritonitis and hypovolaemia was frequently present. Eleven of the 35 episodes were culture-proven peritonitis (5 due to Pneumococcus, 6 due to Gram-negative bacteria) and in 24 the cultures were negative. Hypovolaemia occurred in 6 of the former group and 5 of the latter. The occurrence of these episodes bore no temporal relationship to steroid and cyclophosphamide treatment. Sixty-nine per cent of the complications appeared within the first 3 years of onset of the nephrotic syndrome and 8 of 19 patients experienced multiple episodes. In this study, hypovolaemia always occurred in the context of clinically detected peritonitis and not as a separate complication, suggesting infection together with fluid and protein losses as likely pathogenetic mechanisms.