Why do people fail to engage in positive behaviors which will promote their health and well-being? Researchers addressing this question adopt primarily one of two perspectives, drawing either on theories of health behavior, such as the Health Belief Model (HBM), or on theories of risk perception, such as unrealistic optimism. To overcome this compartmentalization, two studies of cancer screening behavior assessed the extent to which unrealistic optimism occurred in relation to each of the elements of the HBM: severity and curability of cancer and the benefits of, and barriers to, having a screening test. Data were collected using telephone interviews, dialing numbers randomly selected from the telephone directory. In the first study 164 women aged 50 to 70 years responded to questions about breast cancer and screening mammography, while in the second study 200 men aged 45 to 60 years responded to questions about prostate cancer and screening using the prostate specific antigen test. Women had an optimistic bias in relation to breast cancer risk and severity and barriers to having a screening mammogram but not in relation to the benefits of screening. For prostate cancer, there was an optimistic bias for all HBM variables: risk and severity of prostate cancer and barriers to and benefits of screening. It was concluded that unrealistic optimism is broader than perceived risk, being evident for all elements of the HBM.