The broad scope and the seriousness of the burden of illness directly associated with behaviour have been recognized as a major problem of public health and health policy. A good way of obtaining data on and indicators of mental morbidity is to investigate the clientele of general practitioners (GPs). In the overwhelming majority of the patients diagnosed as suffering from a mental disorder or disturbance, both physical and mental symptoms are present. Case registers enable continuous investigation and documentation of changes in the utilization of psychiatric services. In depressions, illness-related behaviour becomes important in terms of how the patient reacts to the mental strain caused by physical illness and the possible threat to his life. Social status and social class are among the main factors influencing mental health and life expectancy. The frequency, severity, first onset, distribution, course, and outcome of mental illness may be subject to environmental factors.