Chlorpropamide-alcohol flush (CPAF) tests were carried out in 15 male and 15 female Type 2 diabetics. Twelve subjects were CPAF-positive and 18 were -negative. The two groups did not differ in age or duration of diabetes, but the CPAF-positive subjects weighed less (mean difference 13 kg) and had higher plasma chlorpropamide levels. There was a negative correlation between plasma chlorpropamide and body weight, and a positive correlation between plasma chlorpropamide and the increase in facial skin temperature. Females had higher plasma chlorpropamide, a greater skin temperature increase and lower body weight than males; there were 11 females and only 1 male amongst the 12 CPAF-positive subjects. The findings confirm that plasma chlorpropamide is a major determinant of the CPAF reaction and also show that body weight strongly influences the chlorpropamide level and, consequently, the outcome of the CPAF test. The sex difference in body weight probably accounts for most, if not all, of the sex difference in the incidence of the CPAF.