Delayed Endolymphatic Hydrops (DEH) is a clinical entity which is characterized by an early phase with a profound sensorineural hearing loss in one ear. After a prolonged period of time (from 1 to 68 years) a late phase of the disease appears with different otologic symptoms. The ipsilateral type of endolymphatic hydrops appears in the deaf ear with consequent episodic vertigo. The contralateral type develops symptoms of endolymphatic hydrop in the previously normal ear, with the onset of fluctuating hearing loss and/or episodic vertigo. This paper is a review of 12 cases of DEH (7 ipsilateral and 5 contralateral, age ranging from 19 to 79 years). Diagnosis was established on the basis of clinical history and the results of audio-vestibular investigations, specifically designed to detect signs of endolymphatic hydrops (including the glycerol-test and neuroradiological imaging of the cerebello-pontine angle). In 8 patients the levels and frequency of anti-collagen I-II-IV-V, anti-laminin autoantibodies, complement, Ig and immuno-complexes were measured. Viral infections (measles, mumps, influenza) were thought to have caused the early phase in 6 cases. In the entire group of 12 patients the delay between the onset of the ear loss and vertigo averaged 12 years. In the group of the ipsilateral DEH, caloric tests showed in 5 cases a reduced or absent response of the deaf ear. About contralateral DEH, caloric tests showed bilateral reduced response in 2 cases, and, in 3 cases, a decreased response only in the previously normal ear. Regarding immunological study, an abnormal level of Ig and Complement was detected in 3 patients.(ABSTRACT TRUNCATED AT 250 WORDS)