The relationship between average monthly air temperature and sex ratios at birth (SRB) was analyzed for children born in Germany during the period 1946-1995. Both the absolute temperature and--more markedly--the monthly temperature deviations from the overall mean were significantly positively correlated with the SRB (P < 0.01) when temperatures were time-lagged against the SRB data by -10 or -11 months. It is concluded that the sex of the offspring is partially determined by environmental temperatures prior to conception.