The article considers phenomenon of medical activity as a determinant of population demand for medical services and needs in medical care. The medical activity is determined as both a criterion of formation of health of patients by force of active using medical care and a criterion of efficiency of functioning of physicians in satisfaction of needs of patients in medical care. The results are presented related to phenomenon of medical activity using methodology of analysis of opinions of patients and level of medical care appealability of population. Medical activity of population and level of satisfaction of demand and needs in medical services is to be considered as an evidence of commitment of population to treatment and prevention. The investigation of these issues acquires particular specificity in conditions of health care system of large municipal agglomeration with developed network of medical organizations of budget insurance, departmental and private forms of property. In these conditions, patients and population are provided with various possibilities of choosing different forms and types of satisfaction of need in medical care independently of cause and motive of using it that is an advantage for implementing medical activity.