Bartonella (Rochalimaea) henselae has been identified as causative agent of cat-scratch disease (CSD). Employing an indirect fluorescence antibody test we found that 20 (100%) out of 20 children with suspected CSD had serum titres to B. henselae of > or = 512. By contrast, in all but one of the controls (n = 332) including subjects exposed to the same cats, patients with diseases other than CSD, and blood donors the antibody titres were < or = 256 (P < 0.001). However, significant regional differences in seroprevalence were noted. CONCLUSION. Serology to B. henselae may supplant traditional criteria for the diagnosis of CSD and prevent patients from unnecessary surgery.