A cross-sectional study design was undertaken to assess pulmonary function in children with insulin-dependent diabetes mellitus (IDDM), and to establish if there is any relationship with diabetic factors and complications. Thirty-eight children (10 +/- 1.8 years) with IDDM and without clinical or radiological evidence of lung involvement, and 41 healthy age-matched reference subjects, underwent a pulmonary function study. Thirteen (34%) of 38 subjects with IDDM were studied at the onset of their disease. Adjusted values expressed as SD score of forced vital capacity (FVC), forced expiratory volume in 1 sec (FEV(1)), and the transfer factor for carbon monoxide (TLCO) were found to be significantly lower than in controls (-0.54 +/- 0.87 vs. 0.40 +/- 1.10, P = 0.0008; -0.11 +/- 0.96 vs. 0.52 +/- 1.07, P = 0.01; -1.60 +/- 1.07 vs. -0.57 +/- 1.28, P = 0.001, respectively). These differences also existed in the group investigated at onset of diabetes. Residual volume (RV) and RV/total lung capacity ratio (RV/TLC) were significantly higher in the whole group of patients with IDDM than in controls (-0.20 +/- 0.83 vs. -0.80 +/- 0.88, P = 0.003; and 26 +/- 6.2 vs. 21 +/- 5.0, P = 0.0002, respectively). Seventeen patients (45%) had abnormal pulmonary function (SD score, less than -1.64): 16 subjects had reduced TLCO, 4 had reduced FVC, and in 3 of the 17, both functional indices were abnormal. There was no significant relationship between pulmonary function indices and diabetic factors or complications. The only significant association was between abnormal TLCO and females (P = 0.03), suggesting that sex may be a predisposing factor for the development of pulmonary complications. This study supports the view that the lung is functionally involved in children with IDDM early on in the course of the disease.