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We estimated variations in growth and reproducibility of lung function and anthropometric variables in cohorts of boys and girls aged 11.5 to 13.5 years at the start of the investigation and followed-up for 5 years at half yearly intervals. In particular, the autocorrelations and their time trends were studied, which gave information on the growth spurt and its variation. In a separate study on 10 boys and 11 girls measurements of lung function were made on five consecutive days to assess the reproducibility. There was fair agreement between the two methods of estimating reproducibilities. Forced vital capacity, forced expiratory volume in one second, standing height and weight were very reproducible and therefore suitable for describing growth from longitudinal data in individuals. Expiratory flows and the thoracic measures were not sufficiently reproducible to be used for constructing individual growth curves.