OBJECTIVE: Child-rearing environments have been associated with morbidity in adult rhesus monkeys. We examine whether such links are also seen with leukocyte telomere length. METHODS: To determine telomere length in leukocytes, blood was collected from 11 adult female monkeys aged 7 to 10 years who had been exposed to different rearing environments between birth and 7 months. Four had been reared with their mothers in typical social groups composed of other female monkeys, their offspring, and 1 to 2 adult male monkeys. The other 7 had been reared in either small groups of peers or individual cages with extensive peer interaction daily. After 7 months, all shared a common environment. RESULTS: Telomere lengths were longer for those adults who had been reared with their mothers in social groups (median = 16.0 kb, interquartile range = 16.5-15.4) than for those who were reared without their mothers (median = 14.0 kb, interquartile range = 14.3-12.7; 2.2 kb/telomere difference, p