The lecithin-sphingomyelin (L/S) ratio is acknowledged to be superior to most procedures for predicting fetal lung maturity in normal pregnancy. In complicated gestations, however, errors have been reported. This study involves 686 normal gestations and 389 pregnancies complicated by fetomaternal diseases. The L/S ratio, creatinine level, and percent of fat-staining cells were measured in samples of amniotic fluid from these patients. The results showed good correlation of all three tests with fetal maturity as measured by weight, Dubowitz criteria, and incidence of respiratory distress syndrome in the normal patients. In the complicated pregnancies, however, the creatinine was unacceptable in up to 30% of the cases. The L/S ratio likewise decreased in accuracy for all parameters of fetal maturity measured. The Nile blue staining of the fetal cells appeared to be the most consistent technic in these cases. A fetal maturity battery comprised of these three assays and other methods of assessing fetal health is advocated in pregnancies complicated by certain disease states.