The study examines the relationship between a leader’s warmth, dominance and three different indicators of leader effectiveness in the environment of a four-month long Managerial Simulation Game. Data about 184 CEOs were gained via self-reports, from the results of the simulation game, and from 3,340 of their followers. Each CEO completed an Interpersonal Checklist (ICL) and was evaluated, on average, by 18 followers in terms of her leadership emergence and perceived leader effectiveness. Group performance was assessed based on the results of game companies. Neither leader warmth nor leader dominance correlated with any of the three indicators of leader’s effectiveness. The analysis of moderation effect however revealed a significant effect of interaction of leader warmth and dominance on group performance, perceived leader effectiveness, and leadership emergence. CEOs with greater warmth are less effective when possessing a low degree of dominance. On the contrary, the relationship between warmth and leader effectiveness is positive when the degree of dominance is high. An effective leader therefore has to be both warm and dominant. A low degree of dominance combined with excessive warmth might yield a counterproductive effect. The results of the study elucidate an underexplored relationship between leader warmth and effectiveness and illustrate the importance of examining various antecedents of leadership simultaneously.