Frequent occurrence of 11p deletions has been reported for diverse types of human cancers including lung cancers and Wilms' tumors. In contrast to the well documented identification of preferential retention of the paternal allele in Wilms' tumors, no data have hitherto been available for cancers of adults. Taking advantage of the paternal allele-specific expression of IGF2 in the normal lung, we examined 79 lung cancer cases to investigate allelic loss at 11p15 and the parental origin of the retained alleles. While 11 of 36 (31%) informative lung cancer cases exhibited 11p15 deletions, only seven of these (64%) retained the paternal allele in tumors (P = 0.274), showing a contrast to the strong paternal bias in childhood tumors. Our strategy eliminates the need for parental DNAs for the determination and should be applicable to other adulthood tumors carrying 11p deletions such as breast and bladder cancers.