PIP: Data from individual birth records for 140,000 live births occurring in New York City in 1968 provide evidence that the folkloric beliefs in the moon's influence on human reproduction found in many societies may have a factual basis. Records were arranged sequentially by day of occurrence and time series were constructed for total births and for 5 categories of spontaneous births: total, female, male, black, and white. A Fourier spectrum was calculated for each time series. Cross amplitude spectra were computed for 2 pairs of independent time series, male and female and black and white births, to minimize the effects of weekly variation, and cross correlations were calculated between a time series generated for the study a sine wave at exactly the frequency of the lunar cycle with its maximum value assigned to the 3rd quarter) and each of the other 6 series. A small but systematic variation of births over a period of 29.53 days, the length of the lunar cycle, with peak fertility at 3rd quarter, was found in the 4 independent time series and remained after weekly variation was removed. The finding itself provides little guidance as to the timing or nature of the moon's influence, but recent research suggests a connection between menstrual regularity and light. The timing of the fertility peak at 3rd quarter suggests that the period of decreasing illumination immediately after full moon may precipitate ovulation.