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This paper documents a strong seasonal pattern in month of first menstruation among U.S. females, and searches for explanations of the pattern. Most previous research on seasonality of menarche has occurred using European data, where peaks have been observed in both summer and winter. Elevation, light, and urban/rural status have been suggested as possible explanatory variables. Frequencies of self-reported month of menarche are computed for 6000 women in the National Longitudinal Survey of Youth. When these data were disaggregated by race, June/July remained the peak for menarche across Whites, Blacks, and Hispanics. This consistent pattern begs for an explanation. Correlations between a menarche dummy variable and other variables from three different domains were computed: Physical characteristics, personality measures, and family characteristics. Several of these correlations were significantly different from 0, but none were large enough to be impressive. Kinship structure in the NLSY dataset were used to partition variability into genetic and shared environmental sources. In this analysis, females who were more closely related to one another were more likely to be similar in their menarche seasonality. In summary, the study documented a strong summer peak in first menstruation. Kinship patterns suggested a biological basis. Correlations with physical, personality, and family variables were trivially small.