Epidemiological studies on neurobehavioural effects in childhood are highly sensitive to selection bias. A study on the significance of low-level lead exposure was performed among school children in the first grade in Aarhus in 1982–83. Deciduous teeth were collected and analysed for the circumpulpal dentin lead concentration. On the basis of the lead levels, a case and a control group were identified. Information on socioeconomical status and past medical history was obtained for all children. A “common sense” stratification of the reason for non-participation, showed bias in particular due to non-responders, while “refusers” caused little, if any bias. A non-participation of 7% significantly distorted the main result of the study in the direction of the null-hypothesis. Current studies of low-level lead exposure vary greatly with regard to non-participation and the extent of selection bias may contribute to understand the apparent diversity of findings of studies in this field.
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