Background: Several laboratory studies have suggested that many people favor potentially harmful omissions over less harmful acts. The authors studied the role of this omission bias in parents' decisions whether to vaccinate their children against pertussis. Methods: Two hundred mail surveys were sent to subscribers to a magazine that had published articles favoring and opposing pertussis vaccination. Subjects were asked about their beliefs about the vaccine and the disease, and whether they had vaccinated their own children or planned to, and they were given test items to identify omission bias in their reasoning. Results: One hundred and three subjects (52%) responded to the survey. Respondents who reported they did not or would not allow their children to be vaccinated (n = 43; 41 %) were more likely to believe that vaccinating was more dangerous than not vaccinating (p < 0.001 ). They were also more likely to exhibit omission bias (p = 0.004), holding constant their stated beliefs about the danger of the vaccine. Conclusions: Omission bias plays a role in decisions not to vaccinate with pertussis vaccine, beyond the role played by belief about the risk of vaccination. Key words: behavioral sciences; decision making; decision theory; ethics; health policy; judgment; pertussis vaccine; probability; psychology; public health; risk; vaccination; whooping cough. ( (Med Decis Making 1994;14:118-123).