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Retinoblastoma is a rapidly growing, usually inherited tumor originating in retinal germ cells. The tumor may invade the brain and spread to distant sites. It is the most common primary malignant eye tumor in children. The usual pattern of inheritance is autosomal dominant; parents with the gene have a 50 percent chance of passing the mutant gene on to their children. About 10 percent of those carrying the mutant gene do not develop the tumor. Retinoblastoma may spontaneously regress, resulting in normal vision, so that the disease may remain undetected. Retinoblastoma may occur sporadically without obvious inheritance; parents of these cases should be examined for regressed retinoblastoma scars. A case is presented of a family where the phenotype segregated with unusual characteristics. The phenotype was expressed in varying degrees ranging from asymptomatic carriers of the gene or regressed tumors, to unifocal and bilateral multifocal tumors. Initial genetic counseling was difficult because of the incomplete penetrance (frequency of manifestation of a genetic feature) and complete ophthalmological examination of the seemingly normal family members was not performed. Using DNA probes, regressed tumors were shown in founder members of the family; this confirmed inherited disease. The pedigree of this family reinforces the need for detailed genetic analysis using DNA probes as well as ophthalmological examination of close relatives. (Consumer Summary produced by Reliance Medical Information, Inc.)