We have karyotyped eight sporadic osteocartilaginous exostoses (OCE), a tumor type not characterized cytogenetically before. Five tumors had only normal karyotypes, whereas three displayed the following abnormal karyotypes: 46,XY,del(8)(q24.1); 46,XX,del(8)(q22), t(8;14)(q24.1;q32); and 46,XY,der(8)t(1;8)(q21;q24),inv(12)(p11q13). All three aberrant cases thus had structural rearrangements leading to loss of the distal part of 8q. This is of particular interest because multiple OCE are part of the disease phenotype in patients with the autosomal dominant tricho-rhino-phalangeal syndrome type II (TRP II), many of whom have constitutional loss of genetic material from 8q24.1. We hypothesis that band 8q24.1 harbors a tumor suppressor gene, the homozygous inactivation of which is important in the genesis of both inherited and sporadic OCE. In the familial form, i.e., in TRP II, loss or functional inactivation of one allele is inherited and only the second mutation is due to a somatic event, whereas both mutations are somatic in the sporadic forms. This hypothesis can be tested by analysis of sporadic and inherited OCE for homozygous loss of 8q24 material with molecular genetic techniques.