Thirty cases of Haemophilus influenzae pneumonia with clinical and laboratory features have previously been recorded in adults. During the past three years, we have examined 18 patients in whom this diagnosis was established by transtracheal aspirate or blood culture. Our study suggests that H influenzae, both typable and nontypable strains, is a more frequent cause of pneumonia in adults than previously appreciated. We found no clinical values that distinguished H influenzae pneumonia from other bacterial pneumonias. A properly performed Gram's stain of a transtracheal aspirate specimen is classical in its appearance and facilitates instritution of appropriate initial treatment. The emergence of both typable and nontypable organisms resistant to ampicillin makes it important that organisms be isolated from reliable samples for sensitivity testing. With appropriate therapy, the prognosis for patients with H influenzae pneumonia appears to be good.