Summary The impact of albendazole (400 mg) and praziquantel (40 mg/kg body weight) treatment of schoolchildren was compared with placebo according to the presence of anaemia (haemoglobin concentration < 11.0 g/dl) and heavy (> 5000 epg) or light (< 5000 epg) hookworm egg load. The study was conducted in rural Tanga. Medication was administered in September 1994 and children were followed-up in January 1995. Overall, anthelminthic treatment reduced the fall in haemoglobin concentration compared with that observed in the placebo group (− 0.11 g/dl vs. − 0.35 g/dl; P = 0.02). Anthelminthic treatment was of greatest benefit to the 9% of children with both anaemia and heavy hookworm egg load (+ 0.67 g/dl vs. − 0.67 g/dl) and was also of significant benefit to the 38% of children with anaemia and light hookworm egg load (+ 0.07 g/dl vs. − 0.21 g/dl). It was of no significant benefit to children who were not anaemic. This study suggests that single-dose anthelminthic treatment distributed in schools in this area achieves haematological benefits in nearly half of children infected with S. haematobium and geohelminths (37% of total population).