This is the first report that the authors are aware of concerning fluctuating visual field defects in patients with Rathke's cleft cysts. Among 11 patients with histologically proven Rathke's cleft cyst, three patients presented with a visual field defect which spontaneously resolved. The symptom became overt again in the subsequent 1 to 14 month follow-up period and a surgical procedure was then chosen. Magnetic resonance imaging (MRI) observation clearly revealed change in the cyst size without signal intensity change in two cases. In the remaining case, multiple spotty high intensities on T1-weighted images were found corresponding to fluctuations in the clinical course. In the last case, enlargement of the cyst was considered to be due to repeated haemorrhage. In the other two cases, minor haemorrhage, undetectable on MRI, may have played a role in the increased cyst size. Another possible mechanism is an imbalance between secretion from, and absorption into, the cyst wall. The size change and neurological fluctuations were previously unknown aspects of the natural history of this lesion. Surgical indications for such patients with transient symptoms followed by cyst size reduction have been controversial, since pituitary cystic lesions may enlarge again or disappear. Our observations suggest that this change in cyst size may be more frequently noted in cases with iso- or high intensity cysts on T1-weighted images, indicating the need for closer observation of patients with this MRI finding.