Stability and change in physical and psychological aggression was examined over a 2 1/2-year period for at-risk young couples in two groups–men who stayed with the same romantic partner and men who had two different romantic partners. Predictions were based on a developmental-interactional model of the course of aggression over time in the young couples. It was predicted first, that there would be significant persistence in any physical aggression across time in the group of couples who stayed together; second, that stability in levels of aggression toward a partner would be higher for young men who remained with the same partner compared to men who repartnered; third, that increases in levels of aggression would occur over time for couples with the same partners; and fourth, that changes in aggression over time would be concordant for couples. Measures of aggression included reports of aggression by both partners and observed aggression. Findings indicated considerable stability in aggression for the same-, but not for the different-partner group.