33 consecutive patients operated upon because of an at the ENT Department of the Acoustic neuroma University of Zürich between 1969 and 1972 were investigated in order to find the most suitable criteria for early diagnosis of the tumor. A unilateral progressive sensorineural hearing loss with or without tinnitus was the first symptom of an acoustic neuroma in 91% of the patients. The most reliable diagnostic tests were rated as follows: (1) the caloric test, presenting a significantly reduced caloric response of the affected side in 94% of the cases and an abnormal directional preponderance of the caloric nystagmus in the remaining patients; (2) the conventional X-ray examination (Stenvers views and polytomography), showing an asymmetry of the internal auditory meatus in 94% of the instances; (3) meatocisternography, which confirms the presence and indicates the precise extension of the tumor in 90% of the cases; (4) the CSF-protein, which is increased above normal values in 81 % of the cases, and (5) audiometry, indicating the presence of a retrocochlear lesion in 55 % of the patients. Attention is drawn to the fact that early diagnosis of an acoustic neuroma does not only mean detection of a tumor in its earliest stage of development but also diagnosis of a lesion when it produces its first symptoms.