Alterations of the retinoblastoma (RB1) tumor suppressor gene are not only associated with retinoblastoma but also with several other malignancies including osteosarcoma. Besides direct sequence alterations, hypermethylation of a CpG island in the promoter region can cause inactivation of the RB1 gene as it has been shown in retinoblastomas. We examined the methylation status of the RB1 gene in 25 osteosarcoma specimens by using the methylation-sensitive restriction enzymes SacII and SmaI. The restriction fragments were hybridized with clone p123, which is a 1.8-kb genomic subclone that spans the RB1 CpG island including the promoter region and exon 1. Whereas we reconfirmed hypermethylation of the RB1 gene in a sporadic retinoblastoma, no hypermethylation could be detected in the 25 osteosarcoma specimens, suggesting that hypermethylation of the RB1 promoter is not of major importance during osteosarcoma genesis.