During a five year observation period, declines of forced vital capacity (FVC) and forced expiratory volume in one second (FEV1) were investigated in 200 subjects with diabetes mellitus (DM), 126 subjects who developed DM during that period and 9,051 nondiabetic subjects. After statistical adjustment for age, sex, height, and tobacco consumption we found that the subjects who developed DM during the observation period had the steepest declines of ventilatory function. Their annual declines of FVC (and FEV1) were on average 29 ml (and 25 ml) greater than the declines observed among the nondiabetic subjects. The subjects who had DM during the whole observation period experienced a decline of ventilatory function which was not significantly greater than the decline among the nondiabetic subjects. Our results suggest that DM, at its onset, is associated with a significantly accelerated decline of ventilatory function. If DM has been present for some years, its impact on the decline of ventilatory function is small.