Morrell, D. (Dept. Clinical Epidemiology and Social Medicine, St. Thomas's Hospital Medical School, London, S.E.1, England), and Kasap, H. S. The effect of an appointment system on demand for medical care. Int.J. Epid. 1972,1:143–151. This paper describes a study in which a number of measurements reflecting the demand for and provision of medical care in a London general practice was made during two seasonally matched periods of six months in successive years. In the intervening period the method of delivering medical care was altered by introducing an appointment system. The study revealed a major shift in medical care in the second period towards the old at the expense of the young, and towards the mentally ill. The difficulty of attributing these changes to the change in the method of delivering care is discussed. The problems presented by differences in the morbidity which populations experience in succeeding years, on sequential studies of this type, are explored.