A statistically significant inverse relationship was found between mean monthly temperature 9 mo. prior to delivery and 7,312 live births in a midwestern community. A curvilinear relationship fit the data more closely than a linear one, suggesting both extremely cold and hot weather were related to the birth rate 9 mo. later. Illegitimate and legitimate births followed nearly the same pattern. Marriages and the number of births 9 or even 10 mo. later were not related. Apparently the weather, particularly cold' weather, has a powerful influence on behavior resulting in conception.