Spirometry was performed on 88 children with insulin-dependent diabetes mellitus (IDDM) and 216 healthy controls living in Sheffield. Children with IDDM had significantly lower percentage predicted forced vital capacity (FVC) than did control children or reference norms. There was no evidence that the reduced FVC was confined to a subgroup of children, and there was no correlation with duration of IDDM or glycemic control. A longitudinal study of 27 children with IDDM did not show progressive deterioration in percentage predicted FVC with age. These findings suggest that a tendency toward reduced lung volumes exists in IDDM and may not be a direct result of the metabolic disturbances in the disease.