There are no reports documenting toxicity or adverse effects after treatment of children aged < 24 months with benzimidazole derivatives and there is an urgent need to clarify this point in light of the potential detrimental effect that soil-transmitted helminthiasis has on this age-group. A total of 653 treatments (317 mebendazole 500 mg; 336 placebo) were administered in 1996/97 to 212 children aged < 24 months as part of a 1-year anthelmintic drug study conducted among preschool-age children in Tanzania. Data on fever, cough, diarrhoea, dysentery and acute respiratory illness were collected 1 week following the treatment. No differences between the occurrence of adverse effects in the 2 groups were observed. In light of the potential nutritional benefit achieved by regular deworming in this young age-group, the policy that excludes children aged < 24 months from treatment should be re-considered.