Continuous subcutaneous insulin infusion (CSII) with a portable pump was compared to unchanged conventional treatment (UCT) in long-term treated patients with IDDM in order to evaluate changes in glucose homeostasis, metabolites, hormones and quality of life. We found that the mean blood glucose values, measured at home, and the HbA1c values were significantly lower in the CSII group compared to the UCT group. The improved control during CSII was followed by a nearly normalization of the diurnal pattern of FFA and ketone bodies in plasma. Plasma free insulin values were significantly higher in the morning (fasting) during CSII compared to UCT, whereas the mean diurnal concentrations and the diurnal pattern were identical in the 2 groups. Both peak values of growth hormone during the day and the fasting values were significantly lower in the CSII group compared to the UCT group. In patients treated with the insulin pump (CSII) the wellbeing (quality of life) was estimated to be significantly improved. Two patients developed ketoacidosis during CSII, whereas 2 controls (UCT) were hospitalized with hypoglycemic coma. We conclude that insulin pump treatment for 6 months results in a near normalization of glucose and FFA metabolism, resulting in an improved quality of life. The improved control seems not to be explained by a change of the diurnal pattern of plasma insulin. However, the higher morning values may be of significant importance.