Fifty seven episodes of severe cytomegalovirus (CMV) infection were treated with iv foscarnet in 13 bone marrow and 33 renal graft recipients. The ranges of the daily dose, duration, average steady state level and total dose were 23-268 mg/kg, 2-46 days, 42-400 mg/l and 2-399 g, respectively. Adverse effects, such as decreased haemoglobin, decreased renal function and increased serum calcium, were observed in a few patients only. Increased liver enzymes, hallucinations and tremor were seen in one uraemic patient and coincided with foscarnet plasma concentrations above 400 mg/l. Among 25 patients evaluated for clinical efficacy, 12 died. Improvements, such as eradication of CMV (8/14 assessable patients), resolution of fever (11/22), and improved laboratory values (13/23) were noted in 17/24 (70%). Controlled trials are warranted on the basis of this study.