OBJECTIVE: To determine the microbial etiology of acute community-acquired pneumonia in Yaoundé. METHODS: Ninety-one consecutive adult patients admitted to hospital for radiologically confirmed acute community-acquired pneumonia were studied prospectively. Sputum microscopy and culture, blood cultures, pneumococcal antigen detection in serum and serologic analysis for agents of atypical pneumonia and for human immunodeficiency virus (HIV) were performed for most patients. RESULTS: There were 65 men and 26 women, mean age 36.5 years. Of 91 patients, 70.3% had at least one predisposing factor and 21.7% (20 of 81) were seropositive for HIV. A microbial etiology was identified in 48 (52.7%) cases. A single pathogen was identified in 42 (85.4%) and double pathogens in six (14.6%). Bacteremia occurred in 12 of 81 patients and was significantly more common in HIV-seropositive than in HIV-seronegative patients. Streptococcus pneumoniae was the commonest causative agent, identified in 22 of 91 (24.2%) patients, 10 of whom were bacteremic. Atypical pathogens were diagnosed in 14 of 65 patients with serologic tests. Mycoplasma pneumoniae and Coxiella burnetii were diagnosed in six of 65 (9.2%) cases each, and Chlamydia pneumoniae in three (4.6%) patients. Mycoplasma pneumoniae and Chlamydia pneumoniae occurred as a dual infection in one case. Seven of 91 patients died, and death was not associated with any particular etiology. CONCLUSION: Streptococcus pneumoniae remains the predominant etiologic pathogen of community-acquired pneumonia. For this reason, and also because ampicillin used empirically to treat patients with this disease in the same setting has been shown to be efficacious, we propose the use of an aminopenicillin in the initial treatment of acute community-acquired pneumonia in adults in Yaoundé. However, patients who fail to respond clinically to such treatment should benefit from either a macrolide or a tetracycline in order to cover for atypical pathogens.