BACKGROUND: Patients can make valuable contributions towards promoting the safety of their health care. Health care professionals (HCPs) could play an important role in encouraging patient involvement in safety-relevant behaviours. However, to date factors that determine HCPs' attitudes towards patient participation in this area remain largely unexplored. OBJECTIVE: To investigate predictors of HCPs' attitudes towards patient involvement in safety-relevant behaviours. DESIGN: A 22-item cross-sectional fractional factorial survey that assessed HCPs' attitudes towards patient involvement in relation to two error scenarios relating to hand hygiene and medication safety. SETTING: Four hospitals in London PARTICIPANTS: Two hundred sixteen HCPs (116 doctors; 100 nurses) aged between 21 and 60 years (mean: 32): 129 female. OUTCOME MEASURES: Approval of patient's behaviour, HCP response to the patient, anticipated effects on the patient-HCP relationship, support for being asked as a HCP, affective rating response to the vignettes. RESULTS: HCPs elicited more favourable attitudes towards patients intervening about a medication error than about hand sanitation. Across vignettes and error scenarios, the strongest predictors of attitudes were how the patient intervened and how the HCP responded to the patient's behaviour. With regard to HCP characteristics, doctors viewed patients intervening less favourably than nurses. CONCLUSIONS: HCPs perceive patients intervening about a potential error less favourably if the patient's behaviour is confrontational in nature or if the HCP responds to the patient intervening in a discouraging manner. In particular, if a HCP responds negatively to the patient (irrespective of whether an error actually occurred), this is perceived as having negative effects on the HCP-patient relationship.