As caries prevalence in some countries declines, and panel sizes for discriminating anti-caries clinical trials inevitably increase, the need for participant pre-selection grows to avoid escalating cost and manpower problems. Hence, retrospective analyses of data were undertaken, from a 3-yr double-blind caries clinical trial where a linear fluoride dose-response had been obtained. As a result, it was demonstrated that the eruption status of permanent second molars, in 3005 participants aged 11-12 yr at outset, when coupled with their baseline clinical DMFT values, provided a sound basis for panellist size prediction without loss of sensitivity. Thus, had only those subjects with DMFT > 4, with at least three of their permanent second molars erupted been selected following clinical baseline examination, the same sensitivity would have been achieved with 52% fewer subjects. Furthermore, using these criteria, selection can be made on site, in real time, and a child informed whether its immediate participation in a trial is possible.