Insulin treatment may improve the outcome of islet transplantation. To determine the effects of insulin treatment on transplanted islets, 4 groups of streptozotocin-diabetic C57BL/6 mice were transplanted with 100 islets, an insufficient beta-cell mass to restore normoglycaemia. Groups 1 (n = 12) and 2 (n = 12), were kept normoglycaemic with insulin treatment from day 10 before transplantation to day 14 after transplantation; groups 3 (n = 12) and 4 (n = 18), were not treated with insulin. Grafts were harvested 14 (groups 1 and 3) or 60 (groups 2 and 4) days after transplantation and beta-cell mass and replication were measured. When insulin was discontinued all mice maintained normoglycaemia; in contrast, non-insulin-treated groups remained hyperglycaemic throughout the study. Fourteen days after transplantation the beta-cell mass was reduced both in group 1 (0.09 ± 0.01 mg) and group 3 (0.14 ± 0.02 mg) compared to the initially transplanted mass (0.22 ± 0.02 mg, p < 0.01); beta-cell replication and area did not change in group 1, but were increased in group 3. Insulin content, expressed as a function of beta-cell mass, was maintained in group 1 grafts (12.5 ± 2.0 μg/mg), but was severely reduced in group 3 (1.0 ± 0.2 μg/mg) compared to non-transplanted islets (20.4 ± 3.3 μm/mg). In group 2, beta-cell mass increased when insulin was discontinued; 60 days after transplantation beta-cell mass was similar to the initially transplanted mass (0.23 ± 0.04 mg), glucose levels after an intraperitoneal glucose challenge were normal, and insulin content was preserved (19.6 ± 2.7 μg/mg). In contrast, beta-cell mass was progressively reduced in group 4 (0.08 ± 0.02 mg, p < 0.001). In summary, insulin treatment reduced the beta-cell mass needed to achieve normoglycaemia in islet transplantation. Islets transplanted to insulin-treated mice showed better beta-cell function, preserved insulin content, and were able to increase their beta-cell mass to meet an increased functional demand. [Diabetologia (1997) 40: 1004–1010]