In a newborn with only minor malformations the finding of an extended interstitial chromosome deletion 13q was unexpectedly found [46,XY,del(13) (q14.11q22.2)]. The included deletion of chromosome band 13q14, which is known to be predisposing for retinoblastoma (Rb), gave rise to subsequent ophthalmological inspection. A multifocal tumor was detected immediately in the right eye and 11 months later contralaterally. In contrast to the Knudson hypothesis, which suggests a high risk of a multifocal and bilateral tumor in patients with an inherited mutation of the RB-1 gene, literature data indicate a reduced tumorigenesis in patients with a cytogenetic deletion of the critical Rb region of chromosome 13. However, the authors' patient shows that even with a cytogenetic deletion early, bilateral, and multifocal tumor formation is possible. Reliable risk estimates of tumorigenesis for patients with a chromosome deletion cannot be given, since most of these were ascertained by their tumor.