We examined stability and change in prosocial moral reasoning (PRM) assessed longitudinally at ages 20/21, 22/23, 24/25, 26/27, and 31/32 years (N = 32; 16 female) using a pencil-and-paper measure of moral reasoning and examined relations of PRM and prosocial behavior with one another and with empathy, sympathy measured with self- and friend reports in adulthood, self- and mother reports of prosocial tendencies in adolescence, and observed prosocial behavior in preschool. Proportions of different types of PRM (hedonistic, approval, stereotypic, internalized) exhibited high mean-level stability across early adulthood, although stereotypic PMR increased with age and hedonistic PRM (a less sophisticated type of PRM) declined over time for males. More sophisticated PMR was positively related to friends' reports of a prosocial orientation concurrently and at age 24/25, as well as self-reports of sympathy in adolescence. Specific modes of PMR related to spontaneous or compliant sharing in preschool. Women used more sophisticated PMR than men across the entire study period. Self-reported and friend-reported prosociality at age 27/28 and 31/32 (combined) was related to numerous prior measures of a prosocial orientation, including spontaneous, relatively costly prosocial behavior in preschool (for self-reports and friend-reported sympathy/consideration for others). Donating/volunteering at T13/T14 was related to concurrent self- and friend-reported prosociality and to self-reported prosocial orientation in earlier adulthood and mother-reported helping in adolescence.