Thrombocytosis is seen in association with many conditions, including infectious diseases. We studied thrombocytosis after severe bacterial infections, particularly pneumonia with empyema in children. A systematic survey of the phenomenon was conducted. Twenty-seven children admitted for pneumonia with empyema were studied. Thrombocytosis (platelet counts greater than 500 x 10(3)/microliters) was present in 92.5%. Platelet counts reached their maximum at 15.1 +/- 3.7 days (range, 7 to 25) and declined to normal after 3 weeks of illness. Compared with a healthy control group, significant thrombocytosis, but of lower incidence, was also noted in children with lobar pneumonia without pleural effusion, bacterial meningitis and osteomyelitis. Platelet functions were examined in seven of the children but no abnormalities were observed. Bone marrow aspiration of three children with pneumonia and empyema showed megakaryocytic hyperplasia. We found no correlation between thrombocytosis, neutrophilia, fever, the clinical course, complications, prognosis or treatment. Neither thromboembolic nor hemorrhagic phenomena were observed.