Non-compliance with drug treatment is widespread. When patients are given medication by their doctors, nearly half do not take the drug or do not take it as prescribed, and most will stop the treatment as soon as they are feeling better. A major problem in identifying the non-compliant patient is the unreliability of many of the measures used for assessing compliance. There are few social and demographic characteristics associated with non-compliance. The type of disease, also, generally has little influence on the level of compliance. Psychological factors such as the patients' levels of anxiety, motivation to recover, attitudes towards their illness, the drug and the doctor, as well as the attitudes and beliefs of significant others in their environment do influence the patients' levels of compliance. Many of the factors that are related to non-compliance with drug regimens are within the control of health care professionals. Contrary to the beliefs of many doctors, studies do not support the view that drug non-compliance is a deviant form of behaviour influenced by patient characteristics.