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A morphological study was conducted on disc material obtained during surgical discectomy in 20 patients. The series was divided into 4 groups. Group I: 3 patients who had never been operated on before; group II: 3 patients who had already been submitted to discectomy but at a level which differed from that of the disc examined; group III: 4 patients treated surgically for contralateral herniation; group IV: 10 patients with true recurrent disc herniation, that is, already submitted to surgery for the treatment of homolateral herniation of the same disc. All of the cases submitted to surgery had been operated on more than a year previously. Moreover, the disc material obtained during autopsy in a patient operated 15 years before for disc herniation was studied. A morphological study of the cases and the data reported in the literature reveals that the fibrocartilaginous tissue of recurrences differs from that of primary herniation because of the presence of large collagen bundles associated with fibrillar framework. This aspect suggests the hypothesis that is also supported by the autoptic material examined that recurrent disc herniation is formed by repair tissue that substitutes the part of the disc that is operated on, and that recurrent herniation is caused by mechanical collapse of the repair tissue. From a surgical point of view, a distinction may be made in terms of early recurrences, which appear within one year of surgery and are formed by disc tissue which is identical to that of primary herniation, and late recurrences, the subject of this morphological study.(ABSTRACT TRUNCATED AT 250 WORDS)