We have analyzed diagnostic efficiencies of the individual "Essential laboratory test" items when these tests were applied to 520 new outpatients in the division of comprehensive medicine in a teaching hospital. The integration of these test results with history-taking and physical examination resulted in 544 primary clinical diagnoses which corresponded to the patient's illness complained and in 361 additional diagnoses unrelated to their chief complaints but found by chance by the addition of the test results. Clinical usefulness of these test items were variable depending on the disease category, demonstrating a superior diagnostic efficiency in infectious or inflammatory diseases, liver and biliary tract diseases, hematological disorders or metabolic diseases such as hyperlipidemia and diabetes mellitus, but a lesser degree of usefulness in gastro-intestinal or neurogenic diseases. Urine urobilinogen could not establish its clinical usefulness because of extremely low diagnostic sensitivity even in liver diseases. The leukocyte differential count provided confirmatory information for infectious or inflammatory diseases and was helpful for the estimation of the etiologic nature of infectious diseases. This study failed to terminate a controversy for the adoption of sialic acid instead of erythrocyte sedimentation rate (ESR) in the "Essential laboratory test" items, since the former test showed lower sensitivity, even though higher specificity, in infectious or inflammatory status than ESR. Low albumin globulin ratio (A/G) revealed equivalent diagnostic sensitivity and specificity to the elevated levels in alpha 1 and/or alpha 2 globulin fractions in infectious or inflammatory status, being helpful for the evaluation of patient's general condition at a glance. Incidental analysis for diagnostic values of cholinesterase and random blood glucose for the detection of fatty liver and diabetes mellitus, respectively, suggested that these two tests may be included in the "Essential laboratory tests". Simultaneous measurement of serum creatinine and blood urea nitrogen levels was recommended for the ambulatory screening of renal insufficiency, rather than the measurement either alone. The results in this study provide scientific bases on the usefulness of the individual test items and should be taken into account in the next version of the "Essential laboratory tests".