The effect of insulin therapy on lung function and lung infections was studied in a retrospective case-control design in 18 diabetic cystic fibrosis (CF) patients; 18 non-diabetic CF patients, matched for sex, age and presence of chronic Pseudomonas aeruginosa lung infection, served as controls. Parameters of CF clinical status were collected for six years before and two years after the onset of insulin therapy in the diabetic patients. Before onset of insulin therapy, body mass index (BMI) and forced vital capacity (FVC) in (pre)diabetic patients deviated increasingly from those in control patients. Decreases in BMI and lung function during the past three months before onset of insulin therapy were reverted within three months of insulin therapy. From three months to two years after onset of insulin therapy, differences in BMI and lung function diminished between diabetic and control patients. After two years of insulin therapy, BMI was similar in diabetic and non-diabetic patients and the percentage differences in forced expiratory volume in 1s (FEV1) and FVC between the two groups were similar to those found six years before the onset of insulin therapy. The finding that insulin therapy improves lung function in diabetic CF patients suggests strongly that the insidious decline in lung function seen during the years before the diagnosis of diabetes mellitus results from the pre-diabetic condition. After onset of insulin therapy, the percentages of sputum examinations positive for Haemophilus influenzae and Streptococcus pneumoniae decreased in the diabetic patients, whereas parameters of lung infections with P. aeruginosa and Staphylococcus aureus remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)