Objective: To investigate the construct validity of Script Concordance Testing (SCT) scores as a measure of the clinical reasoning ability of medical students and practising General Practitioners with different levels of clinical experience. Methods: Part I involved a cross-sectional study, where 105 medical students, 19 junior registrars and 13 experienced General Practitioners completed the same set of SCT questions, and their mean scores were compared using one-way ANOVA. In Part II, pooled and matched SCT scores for 5 cohorts of students (2012 to 2017) in Year 3 (N=584) and Year 4 (N=598) were retrospectively analysed for evidence of significant progression. Results: A significant main effect of clinical experience was observed [F(2, 136)=6.215, p=0.003]. The mean SCT score for General Practitioners (M=70.39, SD=4.41, N=13) was significantly higher (p=0.011) than that of students (M = 64.90, SD = 6.30, N=105). Year 4 students (M=68.90, SD= 7.79, N=584) scored a significantly higher mean score [t(552)=12.78, p<0.001] than Year 3 students (M = 64.03, SD=7.98, N=598). Conclusions: The findings that candidate scores increased with increasing level of clinical experience add to current evidence in the international literature in support of the construct validity of Script Concordance Testing. Prospective longitudinal studies with larger sample sizes are recommended to further test and build confidence in the construct validity of SCT scores.