Incidence, nature and cause of severe acute regional toxicity were studied in 181 patients who underwent normothermic (37-38 degrees C) or 'mild' hyperthermic (38-40 degrees C) isolated limb perfusion (ILP) with melphalan. The known risk factors for toxicity (sex, tissue temperature, blood gas values, isolation level and melphalan peak concentration) were analysed. Severe acute regional toxicity occurred in 30 patients (16%). The limb was painful, swollen, red and warm in 19, often with a smooth and glistening aspect. Blistering scattered over the extremity was seen in 11 cases. In another 11 patients, late blistering limited to the footsole or handpalm developed. Twenty-six patients with severe toxicity had undergone ILP at the iliac isolation level (p < 0.05). Sex and tissue temperature did not predict toxicity. Venous perfusate blood gas values were severely deteriorated in four patients; high calculated melphalan peak concentrations occurred in nine patients. Irreversible long-term morbidity as a sequence of severe toxicity occurred in 10 of the 30 patients. Only one of the 11 patients with late blisters limited to sole or palm developed long-term morbidity (p < 0.05). Thus, the only risk factor for severe acute regional toxicity that could be identified was iliac isolation level. However, in 27 of the 30 patients two or more risk factors were found.