Background: Vitamin A supplementation (VAS) decreases overall child mortality in low-income countries. For logistical reasons, VAS has been linked to routine childhood immunizations. However, several recent studies have indicated that VAS may increase mortality and morbidity from infectious diseases when given with the diphtheria-tetanus-pertussis (DTP) vaccine. The immunological effects of combining the 2 treatments are unknown. Methods: We studied the effect of treating C57BL/6 mice with VAS and DTP, 1 week prior to infection with Plasmodium berghei ANKA. The progression of disease was monitored through parasite load and time to death. Results: We found significantly higher levels of parasitaemia in VAS/DTP-treated mice than in control mice (crude geometric mean parasitaemia ratio 2.02 (1.08-3.76), p = 0.03). There was no effect of administering either VAS or DTP alone, indicating that the increase in parasitaemia was due to a synergistic effect of VAS and DTP (p for interaction = 0.02). The effect of VAS/DTP on levels of parasitaemia was modified by the specific parasite variant used. No effect was observed on time to death. Conclusion: Our results indicate that VAS/DTP can negatively influence the outcome of malaria infection in mice, adding to the concerns about simultaneous VAS and DTP administration to children in low-income, malaria endemic countries.