Determination of the mRNA start site is the first step in identifying the promoter region, which is of key importance for transcriptional regulation of gene expression. The 'oligo-capping' method enabled us to introduce a sequence tag to the first base of an mRNA by replacing the cap structure of the mRNA. Using cDNA libraries made from oligo-capped mRNAs, we could identify the transcriptional start site of an individual mRNA just by sequencing the 5'-end of the cDNA. The fine mapping of transcriptional start sites was performed for 5880 mRNAs in 276 human genes. Contrary to our expectations, the majority of the genes showed a diverse distribution of transcriptional start sites. They were distributed over 61.7 bp with a standard deviation of 19.5. Our finding may reflect the dynamic nature of transcriptional initiation events of human genes in vivo.