A study on the dose-effect relationship between blood lead level and FEP, ZPP, CPU, and ALAU among 250 lead smelter workers was conducted. A nonlinear regression model of a dose-effect relationship between blood lead and porphyrin metabolism indicators was established. The upper limit of normal values of these indicators were estimated from blood lead levels by these nonlinear regression equations. The diagnostic significance was evaluated by multiregression and discriminant analysis. This study would be helpful to the decision-making policy in revising the current diagnostic criteria for occupational lead poisoning.