Between January 1976 and July 1979, 453 cases of malaria were seen at Hillingdon Hospital. The majority of cases were Plasmodium vivax infections in Asians from the Punjab in Northern India-either new immigrants or United Kingdom resident Asians returning from holidays. Twenty-four cases were contracted in Africa or the Middle East. Figures are presented showing a considerable increase in cases during the period of study. In P. vivax infections the time interval between arrival and development of the acute illness was significantly greater for those subjects entering the United Kingdom in autumn or winter.