The predictive value of the preoperative level of bilirubin, alkaline phosphatase and amylase as indicators of choledocholithiasis was determined by prospectively evaluating 304 consecutive patients undergoing cholecystectomy. Elevated levels of bilirubin and alkaline phosphatase are associated with an increased incidence of common duct stones, and the percentage incidence of stones increases with rising bilirubin and alkaline phosphatase levels. Alkaline phosphatase levels as great as 200 are associated with common duct stones in a low percentage of instances, being equivalent to that for unsuspected stones. Levels of 200 or greater are associated with a marked increase in the incidence of common duct stones. An elevated serum or urine amylase level, or both, is of little, if any, value as a predictor of common duct stones. Alkaline phosphatase appears to be a better indicator of common duct stones than does bilirubin, but neither bilirubin nor alkaline phosphatase in themselves are statistically significant indicators. Bilirubin and alkaline phosphatase in combination is a statistically significant predictor of common duct stones at all levels. The combination of a bilirubin level of greater than 3.0 and an alkaline phosphatase level of greater than 250 has a 76.2 per cent probability of an associated common duct stone. The quite important role of operative cholangiography in demonstrating unsuspected stones and in preventing unnecessary common duct explorations is reinforced.