Isolated pleural effusion is rare and occurs when varying degrees of fluid surround the fetal lung without concomitant hydrops. The effusion may regress spontaneously, remain stable in size, or progress to involve both sides of the chest causing fetal hydrops. This may result in pulmonary hypoplasia and fetal or neonatal demise. In this article, we report a case in which spontaneous resolution of an isolated right-sided fetal pleural effusion occurred at 23 weeks of gestation and reappeared bilaterally at 34 weeks. Serial ultrasonographic evaluation of the fetus should be continued even if a spontaneous resolution of a preexisting pleural effusion has occurred. Copyright (C) 2004 S. Karger AG, Basel.