To characterize the defect of insulin secretion in diabetes, the response to different iv glucose loads has been studied in women who have had gestational diabetes and are, by definition, latent diabetic (LD). Women who have produced a large-for-dates baby, but who were not known to have been diabetic (LFD), have been investigated to determine if they have abnormal metabolism. Both groups were found to have raised fasting plasma glucose concentrations. Only the LD had glucose intolerance, which was associated with a reduced first phase insulin response to all glucose loads with a decreased maximal secretory capacity (low V max). The LFD women appeared to include a distinct abnormality in which the β cells had decreased sensitivity to glucose (high Km), with diminished secretory response to small but normal response to large loads. Whereas the LD probably have disordered μ cell function, some of the LFD women may represent the upper end of the normal range of the glucose "set" of β cell function. Neither group had insulin resistance, as measured by the hypoglycaemic response to an iv insulin bolus. A woman who has produced a LFD, but who was not known to be diabetic, does not necessarily have a diabetic tendency.