One hundred consecutive low birth weight (LBW) infants (less than 1751 g) were randomized into a study group having a restricted fluid intake until 4 weeks of age and a control group following the fluid regimen conventionally used in the hospital. Chest X-ray films were examined on admission, at the ages of 3 days, 7 days, 2 weeks and 4 weeks and at bi-monthly visits to the out-patient clinic up to 1 year of age or until the chest examinations were normal. The severity of hyaline membrane disease (HMD) and typical radiological abnormalities of bronchopulmonary dysplasia (BPD) were assessed. Twelve patients succumbed, 1 in the study group and 11 in the control group. The study group seemed to experience less severe HMD than the controls. Of the former 54% and 32% of the latter were alive and had no radiographical signs of BPD at 4 weeks of age (P less than 0.05). The difference between the groups in the cumulative number of normal chest X-ray examinations during the follow up was even more significant. The percentage of normal X-ray films at 1 year of age was 92% in the study group and 72% in the control group. These results suggest that fluid restriction for the first 4 weeks of life can lower the incidence of radiological abnormalities typical of BPD obtained during the 1st year of life in LBW infants. Pulmonary oedema seems to be a significant aetiological factor causing HMD to develop into chronic lung disease.