We wanted to determine the prevalence of pneumonias caused by Legionella species among patients on whom autopsies were performed in two medical centers in St Louis from January 1976 to June 1981. We screened formaldehyde-fixed deparaffinized lung tissue sections with microscopic evidence of pneumonia from 97 patients with use of the direct immunofluorescence antibody technique with a multivalent antilegionella conjugate containing antibodies to Legionella pneumophila serogroups 1 through 4 plus other Legionella species. One patient (1%) had disseminated L pneumophila serogroup 1 infection. We conclude that the prevalence of pneumonias caused by L pneumophila (serogroups 1 through 4), Legionella micdadei, Legionella bozemanii, Legionella dumoffii, or Legionella gormanii is low in the patients studied.