An automated health testing (AHT) laboratory was introduced into a large medical group practice in January 1970. The impact of AHT was studied by means of a matrix of before and after its introduction and the physicians with high and low volumes of referrals to this service. An epidemiologic data-gathering approach was used to obtain information with minimal interference with normal clinic operation during a 3-year period. Data were obtained from the business office, appointment rosters, AHT laboratory results, and clinic records. AHT was accepted and used more frequently by the general and family practice physicians and internists than by the other physicians in the group practice. After AHT was introduced, the general physicians saw fewer patients but held longer visits, performed more procedures, and ordered fewer tests. On the other hand, the internists saw more patients but held shorter visits, performed more periodic reexaminations, and ordered fewer tests. The study findings indicate that in the short term AHT does not appear to reduce the cost of medical care. However, patients benefited from followup of AHT results; often, this followup resulted in significant newly diagnosed diseases or conditions, treatment, and sometimes surgery.