Nephropathia epidemica (NE) is a zoonose caused by Puumala virus. NE belongs to the group of haemorrhagic fevers with renal syndrome. Transient myopia has been described in the acute phase of the disease. This prospective study presents the changes of refraction and the results of ophthalmic A scan measurements of patients who were managed at Savonlinna Central Hospital for NE during an epidemic in the winter of 1992-3. This involved 37 patients and 74 eyes. The incidence of transient myopia was 8.1% and that of myopic shift 40.5%. A scan ultrasound measurements were performed in patients in the acute phase and after total recovery of their general illness. Statistical analysis revealed that there were significant differences in anterior chamber depth and lens thickness between the acute and control phases of the disease and between the patients who had myopic shift compared with those who did not have a significant myopic change in refraction. Based on these results it seems that the reason for transient myopic shift of NE is mainly a combination of two factors: forward movement of the anterior diaphragm and thickening of the crystalline lens. The term myopic shift should be used rather than transient myopia because it better describes the overall refractive change in NE.