Serum analysis of dioxins, furans and PCBs were performed in 1030 adults to identify the determinants of the body-burden of these compounds in the population neighbouring waste incinerators. Despite the use of a sensitive analytical method, several concentrations were not quantified or not detected. To handle this kind of data, we used two different methods: the substitution method and the Tobit model. The results of these two methods were then compared. Results showed that when the censoring intensity is less than 30%, the method for treatment of left-censored data makes little difference. However, when the censoring intensity is larger than 30%, the resulting analysis could be sensitive to the method for handling left-censored data.