To identify critical tumor suppressor loci that are associated with the development of medulloblastoma, we performed a comprehensive genome-wide allelotype analysis in a series of 12 medulloblastomas. Non-random allelic imbalances were identi®ed on chromosomes 7q (58.3%), 8p (66.7%), 16q (58.3%), 17p (58.3%) and 17q (66.7%). Comparative genomic hybridization analysis con®rmed that allelic imbalances on 8p, 16q and 17p were due to loss of genetic materials. Finer deletion mapping in an expanded series of 23 medulloblastomas localized the common deletion region on 8p to an interval of 18.14 cM on 8p22 ± 23.2. We then searched within the region of loss on 8p for loci that might contain homozygous deletion using comparative duplex PCR. An overlapping homozygous deletion region was identi®ed in a 1.8 cM interval on 8p22 ± 23.1, between markers D8S520 and D8S1130, in two medulloblastomas. This region of homozygous deletion also encompasses the 1.4 cM minimal deletion region detected on 8p in ductal carcinoma in situ of breast. In conclusion, we reported for the ®rst time a detailed deletion mapping on 8p in medulloblastoma and have identi®ed a region of homozygous deletion on 8p22 ± 23.1 that is likely to contain a critical tumor suppressor gene involved in the development of medulloblastoma.