The serotonin system has been shown to be involved in the regulation of hostility, anger, and aggressive behavior. Previous molecular genetic studies suggest that the serotonin receptor 1B (HTR1B) rs6296 genotype might have a particular role in these types of behaviors. We examined whether HTR1B is related to hostility, anger, and aggressive behavior phenotypes over a lifespan and whether it modifies the connection between childhood aggressive behavior and adulthood hostility and anger. The participants were 967 women and men from a large population based sample (The Young Finns Study) with a 27-year follow-up. Childhood aggressive behavior was reported by the mother twice when the participants were 3 to 12 years of age. Adulthood hostility and anger were self-reported by the participants between ages 24 and 36. Childhood aggressive behavior predicted adulthood hostility over 27 years. HTR1B SNP rs6296 was associated with childhood aggressive behavior but not with adulthood anger or hostility. The HTR1B SNP rs6296 modified the association between childhood aggressive behavior and adulthood hostility. Aggressive behavior and hostility might form a life course pattern, and the HTR1B might contribute to a development of this pattern.