There has been much debate about reports that some insulin-treated diabetic patients lose awareness of hypoglycaemic symptoms on changing from porcine to human insulin. In a double-blind, crossover study, we sought differences between porcine and human insulin in the frequency and characteristics of hypoglycaemic episodes among patients who reported a reduction of awareness of hypoglycaemia after changing treatment. We studied 50 patients referred by their physicians because of complaints of lack of awareness of hypoglycaemia on human insulin. They had had diabetes for a mean of 20 (SD 12) years and 70% had good or acceptable glycaemic control. Each patient was treated in a double-blind manner for four 1-month periods, two with human and two with porcine insulin, in random order. Only 2 patients correctly identified the sequence of insulin treatments used; 8 or 9 would have been expected to do so by chance alone. The mean percentage of hypoglycaemic episodes associated with reduced or absent awareness was 64% (SD 30%) for human insulin and 69% (31%) for porcine insulin. We could find no statistically significant differences between the insulin species with respect to glycaemic control or the frequency, timing, severity, or awareness of hypoglycaemia. Reduced hypoglycaemia awareness is common with both human and porcine insulins.