A severe case of boutonneuse fever was reported which was only diagnosed after death. The infectious syndrome and cutaneous eruption were associated with meningitis, encephalitis, hypoxaemia and thrombocytopaenia. The absence of an initial lesion ("tache noire") and the notion of recent German measles contamination explained the late diagnosis. Serious cases of boutonneuse fever were usually rare; they were better known as Rocky Mountain spotted fever, a rickettsial infection of the same group, the clinical symptoms of which were very similar and which gave the same proteus agglutination reactions as with boutonneuse fever. An earlier diagnosis, now possible thanks to immunofluorescent techniques using skin biopsies, should enable earlier treatment.