General practitioner prescribing is a subject of legitimate interest to all those concerned with the quality of patient care. The analysis of prescribing and cost data can reveal much about this quality. Suggests that both general practitioners and those who administer and advise on family practitioner services will need to understand the issues involved. Warns that, taken in isolation, prescribing data can be misleading, and it must be compared with other aspects of patient care. Simple calculations can suggest areas of possible under-prescribing and over-prescribing, and in turn lead to consideration of the criteria for diagnosis of common chronic diseases such as asthma and diabetes, as well as their subsequent management. Suggests that the prescribing of both new drugs and those of dubious merit is a subject for particular scrutiny, and those who prescribe these drugs must accept the duty of extra vigilance this imposes on them.