Retinoblastoma (Rb) is an uncommon childhood tumor of the neural retina with a significant genetic component in its etiology. A small proportion of patients have a deletion in chromosome 13 encompassing band 13q14, an observation which permitted the assignment of the RB1 locus to this region. About 20% of Rb tumors exhibit microscopic deletions of band 13q14 or monosomy 13. Trisomy 1q and i(6p) have also been reported in a high percentage of tumors. We analyzed the chromosome complements from direct preparations of 10 Rb tumors derived from seven patients. Modal chromosome numbers ranged from 45 to 48, and occasional duplications of the genomes were noted. In general, the tumors were chromosomally stable, although karyotypic evolution and random chromosome loss were encountered. Consistent abnormalities included trisomy 1q, i(6p), 6q-, and del(13)(q12----14). One patient with bilateral Rb had three tumor clones (two in one eye and one in the other) with chromosome abnormalities unrelated in origin. A second patient with unilateral Rb had two tumor clones with chromosome abnormalities again unrelated in origin. These two patients provide some of the first cytogenetic evidence for the multifocal origin of primary Rb. In the untreated tumor of a third patient, a homogeneously staining region (HSR) was detected in 1p32, indicating gene amplication in vivo; previously, an HSR at this site has been reported in the established Rb cell line Y79.