CAVHD and CAVH were compared regarding uraemic control in 13 critically ill intensive care patients with acute renal failure (ARF). Patients' mean age was 60 years. Pretreatment blood urea range was 17-56 mmol/l (33-56 mmol/l in 70% of the patients). All patients received vasopressor drugs, and 92% were on artificial mechanical respiration. From the results of this study both modalities gave adequate uraemic control. There was a notable decrease in the urea and creatinine levels with CAVHD more than with CAVH. There was no statistically significant difference between the two modalities in the urea clearance. However, a significant difference (P < 0.05) in creatinine was obtained with CAVHD. Our results suggest that CAVHD is a useful alternative to CAVH in ARF especially when the blood urea level is > 30 mmol/l. However, a higher dialysate flow rate (e.g. 25 ml/min) should be used if the urea level is more than 40 mmol/l. CAVH should be reserved for ARF patients in whom fluid overload is a major problem.