The incidence of clinical pulmonary manifestations during clinically mild Plasmodium falciparum malaria was studied in 50 patients. In nine patients (18%), respiratory symptoms developed and consisted of cough either productive (in 5) or dry and pleuritic (in 3), wheezing and dyspnea (in 2). Physical examination of these patients disclosed minimal decrease of breath sounds with diffuse moist rales over both lung bases. Chest X-rays showed small infiltrates and increased vascular markenings in most. Peak expiratory flow rates were measured in 38 of these patients and showed a mean decrease of 16.9% which reached its nadir on the third to fourth day of disease with return to normal values within 7.7 days. In patients with pulmonary symptoms a marked decrease in PEFR was observed (28.9%) and return to normal values was also longer (9.6 days). We conclude that mild, easily detectable and asymptomatic alterations of pulmonary function are observed in most patients with P. falciparum malaria and the incidence of respiratory manifestations in the uncomplicated forms of the disease is relatively high.