When comparing a potentially exposed group to dioxins to a referent group, it is crucial to identify all the confounding factors. In the French Dioxin and Incinerators Study, serum analyses of PCDDs, PCDFs and PCBs were performed in 1030 adults randomly selected to identify the determinants of the body-burden of these compounds in the population living close to waste incinerators. The impact of age, body mass index (BMI) and other potential confounders on the serum levels of all these compounds was assessed. Many factors were relevant for determining current serum levels of dioxins: age, BMI, gender, recent change in weight, smoking status, occupation, location of residence, background food intake, burning activities, and recreational activity. For example, we observed an average increase of approximately 0.3 pg TEQ PCDD/Fs/g lipids per year of age. We recommend that future studies consider all these factors when comparing a group potentially exposed to dioxins to a referent group to avoid misinterpretation. (R.A.)