The causes of the wide spectrum of severity in malaria have only partly been elucidated. There are theoretical reasons for thinking that the infecting dose may influence the severity, but evidence is scarce. We have analysed the records of 82 non-immune neurosyphilis patients bitten by a known number of mosquitoes infected with one of 3 strains of Plasmodium falciparum, whose treatment was delayed. After controlling for strain, the number of mosquitoes was not associated with the prepatent period nor with any of the outcome measures. For one of the main strains, patients with shorter prepatent periods were more likely to receive treatment during the acute phase of the infection, but no other association with measures of severity was found. This study suggests that infecting dose is unlikely to be an important determinant of severity.