Thirty-four of 51 (67%) primary retinoblastomas were analyzed cytogenetically to characterize the type of events that result in additional copies of the short arm of chromosome 6 and their implications in this malignancy. Of the 34 tumors studied, additional copies of 6p were found in 14 (41%). The most frequent mechanism involved to produce additional 6p chromosomes was the isochromosome i(6p) (65%). Other mechanisms were translocations of 6p to other chromosomes (14%), tetrasomy 6 (14%), and additional derived 6q- (7%). Although i(6p) is considered a chromosome rearrangement almost exclusive to retinoblastoma, its significance remains unknown in the carcinogenesis or the progression of retinoblastoma. Our work suggests strongly that the presence or absence of additional copies of 6p defines two categories of retinoblastoma; additional 6p is associated with an undifferentiated histologic degree and invasion of the optic nerve.