Patient adherence, sometimes known as patient compliance, is essential to the success of disease management. Yet close to a quarter of patients consistently fail to follow their physician's recommendations. For six decades, researchers have attempted to understand the factors that predict adherence. Patients' belief about the severity of their condition and its treatment is considered central. This review of the published literature considers the relationship between patients' adherence to physicians' treatment guidelines and patients' beliefs in the threat of their disease, rated health status (self and physician) and the objective assessment of disease severity. This meta-analytic work revealed that better patient adherence is associated with objectively poorer health only for patients whose disease conditions are low in seriousness. On the other hand, patients suffering from serious illnesses, such as AIDS, cancer and heart disease, are less likely to adhere to a physician's guidelines whether the patient's health is rated objectively or by themselves and their family. It may be that when patients are seriously ill many physical, psychological and practical limitations will surface and disrupt a person's desire to follow their physician. The findings suggest the importance of health education, persuasive messaging and the enhancement of patients' health literacy in promoting better adherence.