We prospectively studied 56 patients with an acute ankle injury and an effusion to determine the significance of the ankle effusion to functional outcome. The average size of effusion was 15 mm. Most injuries occurred following a fall (73%) and most (73%) were inversion injuries. At 6 months all but two patients had returned to their normal activities with no further problems. No correlation was found with size of effusion and the functional and clinical outcome. We conclude that an ankle effusion does not reliably distinguish which injuries require further investigations and unless there are clinical indications further investigations are not indicated.