A pilot echographic study was conducted to determine the incidence and severity of optic nerve swelling in acute 'idiopathic' optic neuritis and to examine cerebrospinal fluid dynamics in the subarachnoid space, employing the method of standardised echography and the '30 degrees test'. An attempt was made to correlate the degree of nerve swelling with the initial visual loss and with the rate and extent of recovery of vision. The visual function and echographic features of the optic nerve in 27 patients with the diagnosis of acute optic neuritis were assessed with standardised echography. A significant increase in nerve diameter was found in 74% of cases. There was a correlation between nerve swelling and the severity of initial visual loss. The authors conclude that standardised echography is a useful tool in the diagnosis of optic neuritis and may play a role in predicting the visual outcome.