Determination of fetal lung maturity by measurement of the fluorescence polarization (P) value of the amniotic fluid at room temperature has become the method of choice in an increasing number of perinatal units because of its simplicity and relatively high predictive value. Nevertheless, its power to discriminate between cases with and without hyaline membrane disease (HMD) needs improvements. To this end, we assessed the discriminative power of the P value at the physiological temperature of 37°C (P37) as compared with the power at 25°C (P25). The study group consisted of 288 consecutive cases at risk for preterm delivery. Samples from all 288 cases were measured at 25°C and samples from 112 of these were measured concurrently at 37°C as well. HMD occurred in 27 infants of the total group, nine of whom belonged to the subgroup tested at both temperatures. When sensitivity was fixed at 100% the specificity of P37 was 97% as compared to 79% for P25 (p < 0.001). The percentage of cases with infants free of HMD who had borderline P values was also significantly smaller; 1% vs 21%, respectively (p < 0.001). Although data on more HMD cases are needed to establish the precise threshold of lung maturity for P37, we conclude that P37 is a considerably better discriminator for fetal lung maturity determination than P25.