A prospective study of 187 patients with jaundice and 33 patients with unjaundiced cholestasis was carried out to evaluate the value of serum bilirubin, alkaline phosphatase (ALP), aspartate aminotransferase (ASAT), alanine aminotransferase (ALAT), gammaglutamyl transferase (GGT), and leucine aminopeptidase (LAP) in the differential diagnosis between benign and malignant diseases causing jaundice and/or cholestasis. In the patients with malignant disease (n = 60), the mean serum bilirubin and ALP concentrations were significantly higher (p < 0.001) than in the patients with benign disease (n = 160). Serum LAP, ASAT, ALAT, or GGT levels did not show any significant differences. A stepwise discriminant analysis was carried out to evaluate the value of laboratory tests in predicting malignancy. The discrimination function is DF = bilirubin x 0.71 + ALP x 0.58 + ASAT x -0.24 + ALAT x 0.18 + LAP x 0.08 + GGT x -0.22. When the discriminant function was considered as a diagnostic score (DS), the sensitivity of it in detecting malignancy was 58% with a specificity of 89% and an efficiency of 81%. The DS of serum bilirubin and ALP reached the sensitivity of 61% with a 87% specificity and an efficiency of 79%. The post-test probability of malignant disease calculated by in this test combination was 69%. The LR+ was 4.8 and LR- 0.44. In conclusion, serum bilirubin and alkaline phosphatase seem to be the most potential tests of these laboratory tests in distinguishing benign and malignant causes of jaundice and/or cholestasis, high levels being suggestive of malignant disease.(ABSTRACT TRUNCATED AT 250 WORDS)