Lean management (LM) is well established in manufacturing organizations, and LM adoption in service organizations has recently increased. However, we lack research that focuses on the success of lean management implementations in IT organizations (lean IT). This paper contributes to knowledge of the success factors of lean IT implementation and the relative importance of each factor based on the insights of field experts. The experts identified, agreed on, and ranked 12 implementation success factors for lean IT in a Delphi study using the best/worst scaling technique. The most important factors were leadership involvement, change culture and work ethic, employee involvement, and performance management. Factors of intermediate importance were implementation facilitation, training and education, clear vision and direction, long-term focus, communication, and a holistic approach. Least important factors were existing skills, organizational changes/standardization, and financial resources. This paper contributes a more nuanced understanding of the relative importance of lean IT success factors, proposes relationships between them, and comprehensively explains how to use the rigorous best/worst scaling method in a traditional ranking-type Delphi study.