This study utilized 1985 United States National Ambulatory Medical Care Survey data to explore the relationship between the drug prescribing phenomenon, measured by inappropriate prescribing and by prescription volume, and patient age. Inappropriate prescribing was relabelled potentially undesirable prescribing because limited medico-social patient information restricting circumstantial knowledge could result in an oversimplified and unduly critical judgment of prescribing. Categories of potentially undesirable prescribing were developed: 1) more than one drug in the same class; 2) drug classes not to be combined; 3) drugs serving sub-optimal treatment; and 4) drugs and diagnoses not to be combined. Of 67 different generic drugs representing 13 drug classes studied, 242 products were prescribed to the sample of 33,574 patients age 45+ years. Potentially undesirable prescribing was found to be a common problem regardless of type of medical practice and the sex of the patient: it is related to the age of the patient. The adverse effect of visiting a general practitioner is greater for older patients; and the adverse effect of attending a practice in a non-metropolitan area is larger for the older patients.