Callitrichids are small monkeys with high metabolic rates who appear to be susceptible to spontaneous diseases and possibly to environmental changes creating challenges in maintaining them in captivity. This study investigates whether life events (i.e. medical, social and housing changes) can influence weight. In previous research, body mass has been shown to be correlated with periods of illness, group composition changes and stress. Weights of 56 individual callitrichid monkeys (20 marmosets, 26 tamarins and 11 lion tamarins) at the WCS's Bronx Zoo were examined over approximately 2½ years. Weight fluctuations were scored based on 5%, 10% and 1 standard deviation criteria during periods of medical (illness and injury), social (introductions and separations), housing (movement within or between buildings) events and during periods when no-events occurred. Additionally, weights were examined for 3 months before and after periods of illness to look for trends in weight changes for 47 medical events (14 marmosets, 21 tamarins and 12 lion tamarins). Moreover, in five alloparenting males (four tamarins and one lion tamarin), weights were examined to determine if weight loss occurred after births as observed in earlier studies. The results show that a 5% and 1SD criterion may be too sensitive a criterion. We therefore deemed that a 10% weight loss may be the best criterion. For marmosets, a 10% weight loss occurred in association with all events. For tamarins, weight loss occurred with housing events. In lion tamarins, weight loss was observed with medical events. No significant weight loss was observed in alloparenting males.