The effect of weight loss on immuno function was evaluated in 14 middle-aged obese women. Cutaneous delayed-type hypersensitivity to five recall antigens was measured before a weight loss of 21 kg and again after weight had been stabilized. Weight loss was associated with a decrease in both number and magnitude of delayed-type hypersensitivity responses. The number of positive skin tests decreased from 3.1 +/- 0.2 to 2.4 +/- 0.3 per subject, and the mean cutaneous induration size decreased by 38%. The decrease in induration size was positively correlated with the decrease in body mass index (r = 0.63), weight (r = 0.59), and fat mass (r = 0.52). These preliminary findings suggest that large weight losses may result in decreased expression of immune function.