Urine washing (UW) has been observed in numerous species of prosimians and New World monkeys. The functional significance of UW in Cebidae, specifically, Cebus apella, has not been determined. The objective of our study was to test two major hypotheses related to the function of UW: (1) UW functions as a thermoregulatory mechanism, and (2) UW functions as a means of social communication related to (a) territoriality, (b) sexual encounters, or (c) intragroup aggression/agitation. We collected focal data on a captive group of 28 tufted capuchins (C. apella; July-October 2004 and February-July 2005). We found no significant correlation between UW rates and temperature, at a constant, moderate humidity level. Rates of UW were significantly greater outdoors (no conspecific neighbors) vs. indoors (conspecific neighbors). Qualitative evidence suggests a relationship between UW by the alpha male and sexual solicitations from females. UW rates associated with aggression received were significantly higher than UW rates associated with aggression given and UW rates associated with potential fear/stress. There was also a significant negative correlation between cortisol measures and UW frequencies. Our results suggest that UW does not function in thermoregulation or in territorial communication. Alternatively, our results suggest that UW may be associated with sexual encounters and receiving aggression. Additionally, further investigation is warranted to determine whether UW is used as an appeasement mechanism or as a stress reliever or as both.