To assess possible associations between diabetes mellitus (DM), plasma glucose, forced vital capacity (FVC) and forced expiratory volume in one second (FEV1), we analysed the results from a cross-sectional study of 11,763 subjects. The subjects were 20 yrs of age or older and were representative of the population of Copenhagen City. Two hundred and eighty four of the participants had DM as assessed by questionnaire. One hundred and seven subjects who did not report having DM had a plasma concentration of glucose higher than or equal to 11.1 mmol.l-1. In all age groups of diabetic subjects there was a slight impairment of lung function. It was more prominent in diabetic subjects treated with insulin than in subjects treated with oral hypoglycaemic agents and/or diet. Even in subjects without known DM, there was a significant association between reduction in lung function and raised plasma glucose concentration. On average, FVC (and FEV1) was reduced by 334 ml (and 239 ml) in diabetic subjects treated with insulin, and by 184 ml (and 117 ml) in diabetic subjects treated with hypoglycaemic agents and/or diet compared to control subjects.