The aim of this study was to determine the genetic contribution to the variation in testosterone and the aggression-hostility-anger (AHA) syndrome in middle-aged twins. Moreover, the relation between testosterone and this syndrome, and possible common genetic mechanisms were investigated. Towards this end, blood samples were collected at two time points; the AHA syndrome was measured using three questionnaires: the Buss-Durkee Hostility Inventory with seven subscales, the Jenkins Activity Survey and the Spielberger State-Trait Anger Scale. The results showed substantial heritabilities for testosterone (approximately 60%) and moderate to fair heritabilities for the nine measures of the AHA syndrome (23–53%). The best fitting model for testosterone at two time points included a small age component and additive genetic and unique environmental factors, while a multivariate analysis of the nine AHA subscales resulted in an independent pathway model with two common additive genetic and two common unique environmental factors. No correlation between the common genetic factor influencing testosterone and the AHA subscales was found. We did, however, detect a negative correlation between the common environmental factor underlying testosterone and both common environmental factors influencing the nine AHA subscales, which may reflect a tendency for testosterone levels to rise and hostility to drop (or vice versa) after repeatedly experiencing success (or failure). Twin Research (2000) 3, 266–276.