OBJECTIVE: To develop a validated measure of professionals' attitudes towards clinical adverse event reporting (CAER). DESIGN: Cross-sectional survey with follow-up. PARTICIPANTS: 201 doctors and nurse/nurse-midwives undergoing postqualification training in Leeds, York and Hull Universities in 2003. MATERIALS: A questionnaire which comprised 73 items extracted from interviews with professionals; a second, statistically reduced version of this questionnaire. RESULTS: The analysis supported a 25-item questionnaire comprising five factors: blame as a consequence of reporting (six items); criteria for reporting (six items); colleagues' expectations (six items); perceived benefits of reporting events (five items); and clarity of reporting procedures (two items). The resulting questionnaire, the Reporting of Clinical Adverse Effects Scale (RoCAES), had satisfactory internal consistency (Cronbach's alpha = 0.83) and external reliability (Spearman's correlation = 0.65). The construct validity hypothesis-doctors have less positive attitudes towards CAER than nurses-was supported (t = 5.495; p<0.0001). CONCLUSION: Initial development of an evidence-based, psychometrically rigorous measure of attitudes towards CAER has been reported. Following additional testing, RoCAES may be used to systematically elicit professionals' views about, and inform interventions to improve, reporting behaviour.