It has been suggested that factors which influence low birth weight at term may be associated with reduced lung function in later life. This hypothesis was investigated in a comparative (retrospective) cohort study of 164 matched pairs of subjects where the observers responsible for tracing and studying the subjects were unaware of their case or control status. The subjects, born in Cardiff between 1975 and 1977, were of mean age 15.7 years. Cases (low birth weight (< 2500 g) at term) were matched with controls (normal birth weight (3000-3800 g) at term) for sex, parity, place of birth, date of birth, and gestation. Lung function was measured using a portable spirometer. The corrected mean differences (95% confidence interval) in forced vital capacity (FVC) and flow when 50% or 25% of the FVC remains in the lungs between the cases and controls were respectively -41 ml (-140 to 58), -82 ml/sec (-286 to 122), and -83 ml/sec (-250 to 83). None of these differences were statistically significant. These results are inconsistent with the hypothesis that low birth weight at term is associated with reduced lung function in adolescence.