Pairs of cases of Boutonneuse Fever (BF) occurred in three families. The illness appeared almost simultaneously in both members of each family, but was generally more serious in one as judged by clinical and laboratory parameters. The possibility of a "bed rickettsiosis", that is reactivation of rickettsiae by the blood meal obtained from the first individual by the same tick which fed upon the second individual, could be excluded in two of the three pairs of cases. In only one of the case pairs were the individuals sharing the same bed. The differences in severity of symptoms may be related to the different immunological pattern observed in these patients. Previous rickettsial infection may have provided partial immune protection, as is repeatedly reported in the literature. In one couple, the more seriously ill patient had antibodies of the IgM class, suggesting that this was his first exposure to Rickettsia conorii. The less severely ill patient had antibodies of the IgG class only, presumably as the result of re-exposure after previous asymptomatic infection with spotted-fever-group rickettsia.