We evaluated diagnostic utility of the hematological, biochemical and serological tests comprised in the "essential laboratory tests" advocated by the Japan Society of Clinical Pathology in 1,026 new patients visiting the outpatient unit of Comprehensive Medicine, National Defense Medical College. Of 750 evaluable patients, 52 showed anemia associated with such conditions as ulcer or cancer of digestive tract, inflammatory disease, or renal failure. Leukocytosis (greater than 9,000/microliters) was found only in 25 of 112 CRP-positive (greater than 0.3 mg/dl) patients, suggesting bacterial infection. Forty-four patients showed hypoproteinemia and/or hypoalbuminemia indicating chronic conditions including liver and inflammatory disease. Elevation of serum creatinine level was found in 4 patients subsequently diagnosed with renal failure, whereas 32 patients demonstrated elevated BUN. After application of the "essential laboratory tests", 97 patients were diagnosed with hyperlipidemia (total cholesterol greater than 230 mg/dl and/or triglyceride greater than 250 mg/dl). Determination of serum enzyme activity was useful not only for the diagnosis of liver dysfunction or biliary tract disease but also for those of hematological malignancies or myogenic disorders; however, in patients with abnormal values of LDH, gamma-GT and ALP, clinical significance was not clarified in 53%, 38% and 59%, respectively. These results indicate that the "essential laboratory tests" are useful in the following aspects of primary care medicine: for (1) estimation of the degree or nature of infection or inflammatory status; (2) classification of anemia and its relation to underlying diseases; (3) evaluation of patient general condition and protein-producible function of liver; (4) evaluation of renal function; (5) ambulatory screening for metabolic diseases such as hyperlipidemia; and (6) diagnosis of liver and biliary tract diseases.