An unusual case of acute constriction complicating purulent pericarditis in infancy is reported. When diminished heart size is unassociated with clinical improvement in the treatment of pericarditis with effusion or presumed myocarditis with dilatation, one should consider the diagnosis of acute constriction. Since the physical findings of constriction in infancy may be subtle, definitive diagnostic steps should be considered in this surgically remedial lesion. Inadequate drainage or steroid therapy or both may predispose to the development of acute constriction following acute purulent pericarditis.