A randomized controlled trial in KwaZulu-Natal (South Africa) of 428 primary-school pupils (stratified into 6 groups by age, sex and intervention) measured the effect of different anthelmintic treatments and iron supplementation regimens provided twice at 6-monthly intervals for 1 year (1996/97). Half the pupils received iron supplementation (ferrous fumarate 200 mg weekly for 10 weeks). Pupils received 2 anthelmintic regimens, either (i) albendazole 400 mg plus praziquantel 40 mg/kg or (ii) albendazole 400 mg on 3 consecutive days plus praziquantel 40 mg/kg or (iii) placebo. Baseline prevalences of Ascaris 55.9%, Trichuris 83.6%, hookworm spp. 59.4%, were reduced after 12 months for single-dose albendazole treatment to Ascaris 17.4% (P < 0.005), Trichuris 61.5% (NS), hookworm spp. 0% (P < 0.005), and for triple-dose albendazole treatment to Ascaris 14.8% (P < 0.005), Trichuris 25.0% (P < 0.01), hookworm 0% (P < 0.005). Schistosoma haematobium 43.4% was reduced among treated groups to 8.3% (P < 0.005). There were no significant changes in the anthropometry of the different treatment groups at either 6 or 12 months post treatment. Twelve months after treatment there was a significant increase in haemoglobin levels (P = 0.02) among pupils receiving triple-dose albendazole, praziquantel and ferrous fumarate; pupils receiving no anthelmintic treatment showed a significant decrease as did pupils who received triple-dose albendazole and praziquantel but no iron. Regular 6-monthly anthelmintic treatment significantly reduced the prevalence of Ascaris, hookworm spp. and S. haematobium infections (P < 0.05). Triple-dose treatment for Trichuris was significantly more effective than a single dose of albendazole 400 mg (P = 0.002). In areas with schistosomiasis, hookworm infection and high prevalence of Trichuris infection, combination treatment with praziquantel, triple-dose albendazole, plus iron supplementation, is likely to improve pupils' health and haemoglobin levels.