All-male and all-female groups discussed a case history and provided a statement of their analysis of the case The relationship between leadership style, as measured by Fiedler's Least Preferred Co-worker (LPC) instrument, and task effectiveness (quality of the statement) was investigated for both the top task contributor and the best-liked member of the group LPC of the top task contributor did not relate to task effectiveness. In the role-differentiated groups, LPC of the best-liked member related positively to task effectiveness in the male groups (p < .05) and negatively in the female groups (p < .01) For the male groups, questionnaire data were consistent with the interpretation that the relationship between LPC of the best-liked member and task effectiveness was mediated by the ability of the high-LPC best-liked member to reduce interpersonal tension that interfered with task effectiveness. LPC of die best-liked member of the female groups was related to intermember attraction LPC did not affect role recruitment.