We have studied the infective complications in a group of 75 patients with immunologically-mediated disease who required high dose immunosuppression. There were 22 patients with anti-glomerular basement membrane antibody disease, 19 patients with systemic lupus erythematosus, 18 with wegener's granulomatosis and 16 patients with other forms of systemic vasculitis. The infection rate was 3.69 infections/patient, or 0.74 infections/patient/week of immunosuppression. Bacteria were the commonest infecting organisms (76.1 per cent); serious opportunist viral and fungal infections were less frequent (10.7 per cent) but opportunist pneumonias were an important cause of death. Sixteen patients died (21 per cent) and in 10 of these (62.5 per cent) death was considered to be primarily due to infection. Analysis of six aspects of host susceptibility to infection (age, renal function, dose of prednisolone, cyclophosphamide and azathioprine, and number of plasma exchanges) revealed no single factor as predisposing to infection in the whole group, but in 23 patients who suffered severe infective complications, renal impairment and increasing doses of prednisolone were associated significantly, particularly in combination (p = 0.06). Cyclophosphamide was associated with infection only in the presence of neutropenia, which was rare (13 infections in nine patients). The duration of plasma exchange was not related to the frequency of infection. Fifty patients needed an arteriovenous shunt to provide vascular access for haemodialysis or plasma exchange, and septicaemia occurred in 13; only two episodes of septicaemia were seen in patients without a shunt.