OBJECTIVE: The authors investigated differences between twins in nine pairs of female monozygotic twins in the Australian Twin Registry who were discordant for lifetime bulimia nervosa. METHOD: The twins affected and unaffected by lifetime bulimia nervosa were compared on self-report measures, including a measure of parental bonding, four measures of temperament, and six early-childhood medical conditions. RESULTS: No twins had current bulimia nervosa, and there was no difference in weight or eating status between the affected and unaffected twins. The affected twins reported significantly lower self-esteem and less warmth but more overprotection by their mothers during childhood. CONCLUSIONS: Although limited by the small number of discordant twin pairs and the inability to detect causal relationships, these results suggest that environmental influences that promote low self-esteem may also increase the risk for bulimia nervosa. These temperamental differences may explain the discrepancies in parenting or perceived parenting.