Haemophilus influenzae tends to form part of the usual respiratory flora in adults, especially if they have a chronic underlying disease or are smokers. Pneumonia due to H. influenzae is frequently involved in respiratory infections and its level of resistance to ampicillin has remained stable over the last five years. Most of the literature on the subject was published more than 10 years ago. In this study, we describe the clinical features and evolution of 58 adult patients admitted to hospital for pneumonia due to H. influenzae over a 2-year period, with this group accounting for 6.5% of all the patients admitted with pneumonia during this time period. The etiological diagnosis was made using a good quality sputum sample. Forty patients (69%) were male. The mean age (+/- SD) of the group was 67 (+/-16.8) years and all the patients had at least one underlying disease. The mean duration of the symptoms was 6.7 days. All patients presented an increase in the quantity or purulence of the sputum. On admittance, respiratory failure was present in 52 patients (90%). Gram-negative coccus-bacilli were observed in the direct sputum test and H. influenzae grew in the culture. In two cases, H. influenzae was recovered from the blood culture and in one from bronchial aspiration obtained through bronchoscopy. Another pathogen was identified in 28 patients (48%). In 21 it was another pyogenic bacteria (15 S. pneumoniae, 4 M. catharralis, 1 K. pneumoniae, 1 E. coli), an atypical microorganism in 5 (3 C. pneumoniae, 2 C. burnetii) and a respiratory virus in 2 (syncytial and influenza A). Atypical bacteria and respiratory virus were detected using serological techniques. The radiographic infiltrate was unilobar in 54 of the 58 patients and all showed an alveolar pattern. The empirical treatment included the administration of a third generation cephalosporin (or a fluoroquinolone in patients allergic to penicillin). The evolution was favorable in all the cases in which H. influenzae was the only pathogen or was accompanied by an atypical microorganism or a respiratory virus. Four patients with mixed bacterial pneumonia died (2 S. pneumoniae, 1 E. coli and 1 M. catharralis). The study indicates that pneumoniae due to H. influenzae affects a population with an underlying disease, preferably pulmonary, that it has a longer clinical period than that for pneumococcal pneumonia, that it is slightly bacteremic and, that, usually, it evolves benignly with a low mortality.