In this report of banded karyotypes prepared after short-term culture (72 hr) from human retinoblastoma tumor tissue, on del(13)(pter leads to q14:) chromosome and one normal chromosome #13 were found in all of the metaphases examined. Similar deletions (always involving 13q14) have previously been described in the somatic cells of individuals with one form of retinoblastoma. In the present case, however, the constitutional karyotype is normal. The presence of tumors in both eyes suggests that this is the genetic form of retinoblastoma, even though the patient's family history is negative for this tumor. The normal constitutional karyotype argues that the chromosome deletion occurred as a postzygotic event. The modal chromosome number of the tumor cells is 47 and rearrangements involving chromosomes #2, #17, and #20 were also identified.