It is established that student discipline enhances school outcomes. Despite this fact, student indiscipline is common in secondary schools. Effects of indiscipline have often led to human sorrow through loss of lives and destruction of property. This study investigated the influence of principals' leadership styles on student discipline in secondary schools in Kenya. The study was guided by the Full Range Leadership Model. It was revealed that Transformational and Transactional leadership tend to be negatively correlated to discipline problems while Passive-Avoidant leadership is partially positively correlated to discipline problems. Too many intervening variables existed as to make any reliable conclusion on the effect of principals' leadership styles on student discipline. It is recommended that principals employ all the leadership styles depending on the situation but especially Transformational and Transactional leadership styles for optimal functioning of secondary schools.It is suggested that a longitudinal study on the adaptability of principals to change their styles as they move towards improving the discipline of the schools be conducted.