To determine the long-term efficacy of insulin-pump therapy, we analyzed trends in glycosylated hemoglobin concentrations in 127 patients with Type I diabetes using insulin pumps for periods ranging from 13 to 47 months. In the first year of pump therapy the average glycosylated hemoglobin concentration improved in 83 per cent of the patients, as compared with the value before pump therapy. Although only 11 of the 127 subjects had normal glycosylated hemoglobin values before pump therapy, 33 had a normal average value during the first full year of pump use (P = 0.0001). This favorable trend persisted for the three-year duration of the study. Seventeen of the 19 patients who subsequently discontinued insulin-pump therapy had improved glycosylated hemoglobin values during the period of pump use. Eleven of the 19 patients remained available for follow-up study; the glycosylated hemoglobin concentration became worse in 7 when they returned to conventional treatment. We conclude that the improvement in metabolic control repeatedly demonstrated in short-term studies with the insulin pump can be maintained for a period of years.