OBJECTIVE To evaluate the lamellar body count as a predictor of fetal lung maturity. METHODS We conducted a prospective clinical outcome study. Amniocentesis was performed for evaluation of fetal lung maturity status within 72 hours of delivery in 130 patients. A lamellar body count was performed on each specimen, and a lecithin-sphingomyelin ratio and lung phospholipid profile were performed when possible (insufficient sample or contamination in eight cases). Each infant was evaluated for evidence of respiratory distress syndrome (RDS). RESULTS A lamellar body count exceeding 30,000/microL predicted pulmonary maturity correctly in all cases (negative predictive value 1.00). All 16 cases of RDS had counts of 30,000/microL or less. If the lamellar body count was less than 10,000/microL, the positive predictive value for RDS was 67%, and the likelihood of a mature result from chromatographic phospholipid analysis was low (one of 14, 7%). Values between 10,000-30,000/microL indicated intermediate risk (four of 39, 10%) for developing RDS. Phospholipid analysis indicated fetal lung maturity in 35 of 39 (90%) cases with lamellar body counts in the intermediate risk range. CONCLUSIONS The lamellar body count compares favorably with traditional phospholipid testing in the prediction of fetal lung maturity. Phospholipid analysis is not needed with lamellar body counts greater than 30,000/microL or less than 10,000/microL, but may be of benefit for values in the intermediate risk range. Advantages of this test include speed, objectivity, small sample volume required, and universal availability of instrumentation.