It has become clear that Bartonella henselae is a common cause of cat scratch disease (CSD). The indirect fluorescence antibody (IFA) test for detection of IgG and IgM antibodies to B. henselae concerning CSD showed that 5 (50%) of 10 patients with CSD had a serum IgG antibody titer of 1:128 or more and that 2 (20%) patients had a serum IgM antibody titer of 1:20 or more. One of 7 asymptomatic members of patients' families (14%) had IgG antibody to B. henselae at a titer of 1:256. IgM antibody to B. henselae was not detected in sera from the patients' families. Both IgG and IgM antibodies to B. henselae were not detected in sera from the healthy control group. These data suggest that B. henselae may be a cause of CSD in Japan.