According to Brewerton et al. (1988), CSF 5-HIAA concentrations are at their highest during the summer. When this finding is coupled with the increased rates of battering and rape found during the summer, the results are contrary to the inverse correlation we, as well as others (2, 3), have found between CSF 5-HIAA concentrations and aggression. Dr. Brewerton’s point would be more convincing if CSF 5-HIAA levels were lower, rather than higher, in the summer. Nevertheless, Dr. Brewerton’s point is well taken, and it seems worthwhile to remember to consider the effect of seasonality on both biological and behavioral measures.