We have reviewed 114 episodes of malaria in 110 patients who were admitted to the Infectious Diseases Unit in Leicester during the 5 year period from February 1983-January 1988. There were 71 episodes of vivax malaria (62%), 33 episodes of falciparum malaria (29%), four patients with mixed infection and six patients with negative blood films who were diagnosed on clinical suspicion alone. Most patients presented in the summer months, 68% were aged under 40 years, 39% were born in the Indian subcontinent, 23% in East Africa and 23% in Britain. Eighty-two per cent of patients with falciparum malaria had recently returned from Africa whereas 82% with vivax malaria had visited Asia. Thirty six per cent had been given antimalarial chemoprophylaxis but only half of these took medication correctly. Seventy five per cent of episodes of falciparum malaria presented within 2 weeks of arrival in Britain, however vivax malaria could present at any time and 49% of cases occurred over 3 months after exposure. Presenting symptoms and signs were often non-specific. Twenty nine per cent of patients had been treated with antibiotics and 11% received antimalarials prior to admission. Vivax malaria was generally a mild infection but falciparum malaria was often severe with 39% of patients experiencing complications including one death. Although Plasmodium vivax and P. falciparum are morphologically similar the diseases caused by the two species of parasite are quite distinct. Physicians must ensure that malaria is excluded in anyone who has travelled to an endemic area.