The results of radiological examination of the thorax and abdomen were studied in premature babies with Rhesus incompatibility. In the 53 cases where Rhesus incompatibility was the only abnormal finding, various measurements were made on the radiological images: cardiothoracic index, liver and spleen, distances between the digestive tract loops and the thoracic wall. The same measurements were made in two groups of premature babies, born at the same period of gestation, who acted as controls. The most frequently observed thoracic abnormality was interstitial edema with hypervascularization (50% of the moderate forms) which was present in 14 of the 18 severe cases. Total opacity of both lung fields was noted with the same frequency in the severe forms with anasarca as in the moderate forms without clinical edema. The cardiothoracic index was increased in 40% of the moderate cases and in all severe cases (normal values for premature babies of 30 to 36 weeks: 0.48 +/- 0.04). Pleural effusions were present in 20% of the moderate, and 6 out of 18 severe cases. Abdominal organ measurements in the two control groups were as follows: --premature babies (34 to 36 weeks): --liver: 47 mm +/- 5.3, --spleen: 11.5 mm +/- 2.1, --distance between loops and wall: 7.1 mm +/- 1.4. --premature babies (30 to 33 weeks): --liver: 41 mm +/- 4.7, --spleen: 10.4 mm +/- 3.4, --distance between loops and wall: 6.4 mm +/- 1.2. The babies with Rhesus incompatibility had hepatosplenomegaly which was constant in all severe cases but present in only 70% of moderate cases, an increased distance between the digestive loops and the thoracic wall in all severe forms, and in 15% of moderate forms without clinical ascites. Normal radiological images developed after about 5 days for thoracic signs, and between 7 and 10 days for abdominal signs. Complete recovery occurred in all cases, even severe, except one case, where Rhesus incompatibility was the only abnormality. In 22 cases, Rhesus incompatibility was associated with another disorder: in 12 cases a hyaline membrane was present which radically changed the prognosis (8 deaths secondary to this infection out of a total of 12 deaths). Apart from the study of the Rhesus abnormality, a systematic examination of the size of the liver and the spleen, and measurements of the distances between digestive tract loops and thoracic wall in the first thoraco-abdominal radiological image in a premature baby, can be of diagnostic value for intraperitoneal infections and eflusions of the new-born.