(1) The need for morbidity data based on general practice arises because: (a) the consultation in general practice is the entry point into the health care system; (b) among the health problems brought to the attention of doctors, most are dealt with completely in general practice; (c) general practice records can provide a comprehensive database for health care. (2) The routine measurement of morbidity has to be based on "working diagnostic terms" derived by consensus amongst recorders because: (a) many episodes of illness involve only one consultation and the doctor is required to make the most of the information available to him at the time; (b) the specification of criteria would require validating evidence of conformity and this is not a realistic option on a wide scale; (c) health care data which include the opinion of the general practitioner are more valuable than data based on patient perceptions of illness. (3) The analysis and interpretation of data from general practice: (a) should preferably be based on persons as the unit of analysis; (b) when based on consultations, may be useful for examining workload, but has limited epidemiological value; (c) can, by person linkage, facilitate the study of disease concurrence; and (d) is essential for managing the health care system and monitoring the public health.