This research endeavored to deepen the understanding of the influence, associations and effects of transformational leadership, instrumental leadership and the quality of leader-member exchange (LMX) on effective performance at work. The study proposes and investigates associations between transformational and instrumental leadership styles and the quality of the relationship between leaders and their subordinates, as well as the impact of these leadership styles and LMX on employee performance. The hypotheses were tested through multivariate regression analysis, with data from 310 employees of a medium-sized private Brazilian company in the medical-hospital segment, with more than seven decades of tradition in the national market. The complementary influence of transformational and instrumental leadership styles on the quality of the leader-member relationship (LMX) was verified, as well as the effects of the latter on effective performance as observed through objective metrics applied to monitor employee performance. This research contributes to other studies on the subject, because it deepens the understanding of the phenomenon of leadership, its practical and theoretical implications for organizational management, in addition to strengthening the contemporary national academic literature.