This prospective and consecutive study of 74 patients with completed stroke elucidates occurrence, localization and evolution of lacunar infarcts on repeated CT examinations. Twenty patients had large infarcts (diameter greater than 3 cm), 25 medium-sized infarcts (diameter greater than or equal to 1.5 cm - less than or equal to 3 cm), and 16 had lacunar infarcts (diameter less than 1.5 cm). In 13 patients no infarct was seen. The lacunar infarcts were characterized by delayed appearance on CT, low incidence of fog effect, and infrequent presence of contrast enhancement. In 9 of the 16 patients (56%) the lacunar infarct could be identified on the first CT, performed approximately 3 days after the stroke. In 2 patients the infarct was first revealed on the second (2 weeks post stroke) and in 5 on the third CT (6 months post stroke). The delayed appearance might be due to a partial volume effect. Early development of fog effect may also be considered. As contrast enhancement was observed in only 8 per cent of the patients with lacunar infarcts on CT, and in 70 per cent of the entire group of patients in our series with ischemic infarcts, contrast enhancement seemed to be a function of lesion size.