Restriction enzyme mapping has been used to study the relationship between methylation at the 5' end of the human estrogen receptor gene (ER) and its various levels of expression in estrogen target tissues, like endometrium, placenta and breast. The methylation patterns were compared with that found in white blood cells. A cluster of CpG sites, including the transcription start site, was undermethylated in all examined tissues. In front and downstream of this area, a 1 Kb region was undermethylated in endometrium where the ER gene is highly expressed, methylated to some extent in placenta and breast, which express the gene to a low degree, and greatly methylated in white blood cells which are thought not to express this gene. These data provide evidence that in the 5' end of the human ER DNA, CpG methylation is tissue-specific, and related to the level of the ER gene expression.