The possibility that positive or negative associations occur between the filarial nematode Onchocerca volvulus and the gastrointestinal nematodes Ascaris lumbricoides and Trichuris trichiura was investigated in 205 children attending three schools in central Cameroon. Despite the closeness of their locations, marked differences between the schools were detected with respect to the number of species carried, the prevalence of O. volvulus and T. trichiura and the interaction between the prevalence of these two species. The number of species carried and these same prevalences varied significantly across the narrow age range of the study group. In addition, we observed an interaction between the prevalences of O. volvulus and A. lumbricoides that was dependent upon school but independent of host age and sex. Quantitative analyses revealed that the abundances of O. volvulus, A. lumbricoides and T. trichiura were affected by both school and age whereas host sex was significant only for O. volvulus. Finally, we observed significant positive interactions between the intensities of A. lumbricoides and T. trichiura and between O. volvulus and T. trichiura, that were school, age and sex independent. Our data suggest that associations do occur between filarial and gastrointestinal nematodes and that certain individuals are prone to multiple and high-level infections.