The purpose of this study was to examine the influence of a physician's speciality on the choice of "essential laboratory tests" and their response to the test results. A retrospective case-control study was conducted in divisions of the Department of Internal Medicine in Jichi Medical School Hospital. Twenty-seven parameters were evaluated in 213 outpatients (mean age 47 +/- 17 years; 53% female). Forty-three percent (2443 of 5751) of the tests were conducted by the physician under study and 10.5% (257 of 2443) of their tests were abnormal. The physician's speciality influenced the choice of the ambulatory medical examinations and their response to abnormal tests. Physicians tend to be more sensitive to the abnormality of tests related to their specialty. Hence, it is necessary to constitute guidelines on the response to abnormal "essential laboratory tests".