We describe a case of Legionnaires' disease in a 64-year-old man, in which hairy cell leukaemia was diagnosed after the onset of the infection. Immunological studies revealed a complete suppression of blood monocyte chemotactic and oxidative burst activities. We suggest that in hairy cell leukaemia both monocytopenia and defective functions of monocytes underlie the increased susceptibility to intracellular infections including Legionnaires' disease.