Objective This study aimed at creating electronic cases (e-cases) and analyzing their validity as a diagnostic assessment tool within the context of continuing medical education (CME) to measure general practitioners’ (GPs) knowledge of common infectious disease. Methods We designed assessment e-cases in an electronic CME platform. The e-cases were designed to measure GPs’ knowledge about diagnosis and treatment of common infectious disease in outpatient settings. The data collected were analyzed for five forms of evidence: content, response process, internal structure, relations with other variables and consequences. Results A total of 46 GPs participated in the study. Among them, 87% perceived the e-cases as resembling the patients whom they visit in their everyday practice. Although attendance in this activity made 85% of the participants more cautious about prescription of antibiotics, we could not detect any statistically significant association between the assessment scores and the physicians’ previous antibiotic prescription. The diagnostic assessment with e-cases was supported by most of the elements of validity evidence, including content, response process, internal structure and consequences. Conclusions Overall, evidence suggests that using e-cases might be a valid diagnostic assessment CME activity to measure GPs’ knowledge of common infectious disease, but more research is necessary.