Apart from their effects upon pancreas, sulphonylureas have been shown to influence the insulin action upon extrapancreatic tissues: they increase the number of insulin receptors in peripheral cells, enhance insulin binding to cells and alter postreceptor insulin action. Consistently, sulphonylureas potentiate the impact of insulin in vivo. Recent observations of this kind have renewed interest in the use of sylphonylureas in combination with insulin in diabetes. Several reports have lately been published on the use of combination treatment. Some type 2 diabetics with preserved beta cell function quite obviously benefit from the addition of sulphonylurea treatment to insulin therapy. This has been demonstrated in patients exhibiting secondary failure to oral drugs, and in diabetics requiring large amounts of insulin. Combination therapy does not seem to be effective in type 1 diabetics. The mechanism by which sulphonylureas improve the glycaemic control in diabetics on insulin is not clear. Both enhanced endogenous insulin secretion and potentiation of insulin activity may be involved. Further controlled clinical studies comparing the effects of combination therapy with those of insulin monotherapy are needed before the general utility of sulphonylurea + insulin therapy can be assessed.