The L/S ratio of gastric aspirate was determined in the first day of life in 47 patients with IRDS and in 87 without IRDS. In 97.7% of those without IRDS the gastric L/S ratio was greater than 3.5. Ninety three percent (93%) of the patients with IRDS had a gastric L/S ratio less than 3.5. In these cases the palmitic percentage was 49.1 +/- 11.5 been in normal full term infants 69.4 +/- 5.2 (p less than 0.01). The use of a biochemical score including the L/S ratio and the palmitic acid percentage in patients with IRDS could differentiate survivors and non survivors.