MOTIVATION: Despite the increasing notions of the functional importance of antisense transcripts in gene regulation, the genome-wide overview on the ontology of antisense genes has not been obtained. Therefore, we tried to find novel antisense genes genome-wide by using our LongSAGE dataset of 202 015 tags (consisting of 41 718 unique tags), experimentally generated from mouse embryonic tail libraries. RESULTS: We identified 1260 potential antisense genes, of which 1001 are not annotated in EnsEMBL, thereby being regarded as novel. Interestingly their sense counterparts were co-expressed in the majority of the cases. CONCLUSIONS: The use of LongSAGE transcriptome data is extremely powerful in the identification of thus-far unknown antisense transcripts, even in the case of well-characterized organisms like the mouse.