OBJECTIVE: To determine if group housing affects the variance of body composition parameters in a highly inbred mouse strain. RESEARCH METHODS AND PROCEDURES: Thirty 3-week-old male C57BL/6J mice were obtained from the Jackson Laboratory. Fifteen mice were housed individually, and 15 mice were housed in groups of 5/cage. Animals were fed ad libitum and maintained in the same room under a 12:12-hour light/dark photoperiod at 22 degrees C for 9 weeks. Animals were killed, and fat mass, soft-lean tissue mass, bone mineral density (BMD), and bone mineral content (BMC) were determined by DXA. At necropsy, weights of the paired epididymal fat pads, paired retroperitoneal fat pads, right inguinal fat pad, liver, kidneys, paired testes, and seminal vesicles were obtained. RESULTS: Relative to mice housed singly, group-housed mice showed significantly greater variance in percentage of body fat, testes weight, and BMC. Group-housed mice tended to show greater variance in liver weights and BMD. Mice housed singly were smaller, had less soft-lean tissue mass and BMC, and lower BMD when compared with group-housed mice. DISCUSSION: These results suggest that with respect to body composition parameters, mice housed singly are more similar to one another than are group-housed mice, most likely because of a reduction in environmental (predominately behavioral/social) effects. Thus, mice housed singly may be more representative of genotypic effects on body composition than group-housed mice. Whether other inbred strains of mice show similar responses to housing condition is unknown.