Use of TEM in the diagnosis of opportunistic infections in AIDS patients is indicated in three situations. First, when an opportunistic infection is suspected, but an infectious agent is not visible in tissue sections studied by light microscopy (LM). This situation is illustrated by the following case.(1) A 33-year-old man with AIDS presented with fever, nightsweats, generalized lymphadenopathy, and a 30 lb. weight loss. An inguinal lymph node biopsy was performed. There was no consensus as to the diagnosis among the pathologists who studied H&E sections of the biopsy. An obscure infectious disease, a B-cell lymphoma with amyloid stroma, and possibly Hodgkin's disease were the major considerations. TEM study revealed unexpected clusters of small, pleomorphic, coccobacillary bacteria in vascular walls typical of cat scratch disease. These bacteria were later further identified by polymerase chain reaction as belonging to the genus Rochalimaea(now Bartonella), an important cause of cat scratch disease.