A radiological study of 85 patients with acute pericarditis and effusion included a group of 35 cases observed before the introduction of ultrasonography and a second group of 50 patients in whom the presence of pericarditis had been confirmed by this investigation. The most important conclusions established were the following: --Absence of radiological signs in 55% of cases (group 1 : 46%, group 2 : 60%); normal heart size in 54% (group 1) and 78% (group 2), and increased size in 34% (group 1) and 20% (group 2). --Hilar manifestations (overlapping and obscuring of the left hilar region) in 26% of cases with a clear predominence of the left forms (14 out of 22). The cardiomegaly was not significant in 28% (group 1) and 14% (group 2). --The high frequency of pericarditis with a normal heart size has to be emphasized. The diagnostic value of hilar manifestations is also mentioned; the sign of left hilar overlapping is described in greater detail. --An overall comparison between the two groups shows, more particularly, the equal importance of left hilar manifestations for the radiological diagnosis of pericarditis. In a general way, it would appear that these hilar signs are the only elements which enable objective diagnosis of pericardial effusions on standard films.