Two methods of insulin treatment--intensified conventional therapy (ICT) of three to four daily injections and continuous s.c. insulin infusion (CSII)--were compared in nine patients with insulin-dependent diabetes mellitus (mean age +/- SD 29.4 +/- 5.6 years; duration of diabetes 14.6 +/- 2.9 years). Patients followed each regimen for 3 to 4 months. Under both regimens hemoglobin A1 (HbA1) levels were lower than those recorded previously, but under CSII the mean HbA1 and glucose levels were significantly lower than under ICT (7.8 +/- 0.1 vs. 8.9 +/- 0.2% for HbA1 and 136 +/- 40 vs. 155 +/- 60 mg/dl for blood glucose, P less than 0.001). There was no difference in the frequency of blood glucose levels less than 60 mg/dl, but under CSII hypoglycemic symptoms appeared at lower glucose levels. There was no marked difference between the two regimens regarding scores for depression and anxiety and other psychological parameters, but patients who chose to continue treatment with CSII following completion of the study had previously manifested a significantly higher degree of distress symptoms, which had been reduced during the study. CSII was preferred by some patients because of the superior metabolic control achieved and the greater flexibility in time and size of meals permitted, but was rejected by others because of technical failures and bulkiness of the device. It is concluded that metabolic control can be improved by either regimen, but external pumps must be further miniaturized and technical failures eliminated before CSIII is acceptable to larger numbers of patients.