Upper limb hypertonicity management is a complex domain, in which there is limited high level evidence to support treatment approaches. Therefore, it is important to identify clinical experts to assist practice development. This study aimed to investigate the relationship between the factors related to clinical experience and the development of expertise in this area. Through correspondence, 29 therapists were asked to choose the most appropriate intervention for 60 case vignettes. The responses were examined using the Cochran-Weiss-Shanteau Index of Performance, and were analysed in regard to therapists' ability to prescribe differing interventions based on differences in the vignettes and on their consistency when vignettes were unknowingly repeated. The number of professional development activities that the therapists had undertaken since attaining occupational therapy qualifications was the only factor significantly associated with increased levels of expertise. The factors not significantly associated with increased expertise included therapists' work setting, years of experience and current proportion of caseload comprising hypertonicity. The results indicated that expertise gained in the area of hypertonicity was not specific to clients' age or condition because, although all vignettes presented were those of children with cerebral palsy, there was no significant difference in the expertise found between paediatric and adult therapists. Further research is warranted to investigate the forms of professional development that best support the development of expertise in this area.