Volume and contents of lipid and protein in the pleural fluid from a three weeks old girl with spontaneous chylothorax were studied (a) during parenteral, nonfatty nutrition and later (b) during administration of a formula (Biosorbin) containing medium chain triglycerides (MCT). The pleural fluid production could not be correlated to the treatment employed but suddenly ceased after 20 days management. Triglyceride and total esterified fatty acid concentrations in pleural fluid were high on admission when feeded with human milk, and chylomicrons and other lipoproteins were present in the chyle. During parenteral treatment a pronounced decrease in pleural fluid concentrations of triglyceride and total fatty acids occurred concomitant with a disappearance of the chylomicrons. During the following MCT diet a pronounced increase in triglyceride and total fatty acids concentrations appeared and the chylomicrons reappeared in the chyle. The cholesterol and phospholipid concentrations in the pleural fluid showed only small changes during the different treatments. No significant changes in protein and albumin concentrations of chyle were observed. It is concluded, that administration of the Biosorbin MCT formula, containing 871/2% of the fat as MCT, seems without value ih the treatment of spontaneous, neonatal chylothorax.