Making diagnoses is essential if one is to provide a meaningful service to clients, in sport psychology or elsewhere. Discussion of this topic in the sport psychology literature is rare and is usually limited to the use of standardized questionnaires or unspecified interview techniques. A procedure for the diagnosis of an elite athlete is described and explained as a case study. Critical elements include (a) using an integrative diagnostic procedure where the results of one phase are used to guide the generation of further hypotheses and selection of diagnostic tools, (b) attending to an athlete's strengths and deficiencies, (c) individualizing the procedure and materials and actively involving the athlete where appropriate, and (d) objectively examining interactions between the data at the individual level. Questionnaires, a grid, and a sequential analysis were integrated in phases to refine, confirm, contrast, and clarify points of action to optimize the athlete's performance.