The incidence of bacterial, viral, mycoplasma, and rickettsial infections has been assessed prospectively in 210 adult patients with pneumonia who presented to a district hospital over a six-year period. One hundred and thirteen infective agents were detected in 103 patients. The agent most frequently detected was Mycoplasma pneumoniae which accounted for 30 infections. A bacterial pathogen was found in 43 patients. Streptococcus pneumoniae was the most common of these (24 patients); Staphylococcus aureus (eight), Haemophilus influenzae (four), Klebsiella spp (three), and Legionella pneumophila (three) were all less common. Chlamydial or rickettsial infections (Psittacosis or Q fever) were detected in nine patients. Viral infections were found in 31 patients (22 influenza A, four influenza B, two parainfluenza, and three respiratory syncytial virus). There were 10 patients in whom more than one pathogen was identified. In 107 patients no pathogens could be identified. Seventy-five per cent of these patients had either received antibiotics before entering hospital, or were unable to produce any sputum for culture. The incidence of bacterial pneumonia has probably therefore been underestimated. Nevertheless this survey does emphasise the importance of M pneumoniae as a pathogen in patients with pneumonia presenting to hospital.