The frequency of community-acquired pneumonia coupled with its mortality rate of 10 to 25% is of growing concern to clinicians. A prospective study of 67 patients with severe community-acquired pneumonia was carried out to determine the causative agents, the impact fore-knowledge of the etiology has on the outcome, the value of clinical and radiologic criteria in predicting the evolution, and the efficacy of empirical therapy. The study group included 45 men and 22 women (mean age: 56.8 +/- 16.6 yr), and 46.2% suffered from a concurrent debilitating disease. The cause of pneumonia was diagnosed in 32 cases, and the most common pathogens were Streptococcus pneumoniae (37.5%), Legionella pneumophila (21.8%), and gram-negative bacilli (25.0%). The fact that fungal infections were present in three patients and Pneumocystis carinii in one are worthy of note. The overall death rate was 20.8%. A fatal outcome was related to the age of the patient (p less than 0.05), the presence of debilitating disease (p = 0.026), and septic shock (p = 0.0009). Diagnosis of the causative agents did not aid in increasing the survival rate, but it did allow for better patient management. Most of the patients (85.1%) initiated on treatment with erythromycin plus tobramycin recovered, but only 68.4% of the subjects commenced on treatment with other therapeutics survived. Furthermore, it was necessary to modify the therapy of a greater percentage of the latter group (p less than 0.025). Gram-negative bacillary pneumonia was a frequent finding among the patients who did not recover, making empirical treatment with erythromycin plus third generation cephalosporins most advisable for severe cases of community-acquired pneumonia.