This longitudinal study sought to examine ways in which coaching and mentoring relationships impact on the professional development of nurses in terms of career and leadership behaviours, and evaluating the differences and similarities between those coaching and mentoring relationships. According to the UK government, leadership in nursing is essential to the improvement of service delivery, and the development and training of all nurses is vital in achieving effective change. A coaching and mentoring programme was used to explore the comparative advantages of these two approaches for the leadership development of nurses in acute, primary care and mental health settings. A longitudinal in-depth study was conducted to measure differences and similarities between the mentoring and coaching process as a result of a six-month coaching/mentoring programme. Five nurses from six UK Health Care Trusts were allocated to a coaching group (n = 15) or a mentoring group (n = 15), these were coached or mentored by a member of the senior directorate from their own Trust. Qualitative and quantitative data were collected at three time points (T1 = baseline, T2 = 4 months and T3 = 9 months) using semi-structured interviews and questionnaires. While mentoring was perceived to be 'support' and coaching was described as 'action', descriptions of the actual process and content were quite similar. However, while both groups reported significant development in terms of career development, leadership skills and capabilities, mentees reported the highest level of development with significantly higher scores in eight areas of leadership and management and in three areas of career impact. Implications for nurses and health services are discussed.