Ten unrelated retinoblastoma patients were studied cytogenetically with high-resolution prophase banding; two had a deletion of chromosome 13, band q14. Neither of the two patients had any of the congenital defects usually associated with 13q14 deletions. In patient A, the deletion was found to be de novo. Patient B was found to be mosaic for the 13q14 deletion with 54% of the lymphocytes examined having the deletion. This study indicates that the 13q14 deletion may occur in a significant portion of all retinoblastoma cases. Esterase D activity and isozymes were studied also. The significance of findings will be discussed.