Different immune cells are expected to have unique, obligatory, and stable epigenomes for cell-specific functions. Histone methylation is recognized as a major layer of the cellular epigenome. However, the discovery of histone demethylases raises questions about the stability of histone methylation and its role in the epigenome. In this study, we used chromatin-immunoprecipitation combined with microarrays to map histone H3K9 dimethylation (H3K9Me2) patterns in gene coding and CpG island regions in human primary monocytes and lymphocytes. This chromosomal mark showed consistent distribution patterns in either monocytes or lymphocytes from multiple volunteers despite age or gender, but the pattern in monocytes was clearly distinct from lymphocytes of the same population. Gene Set Enrichment analysis, a bioinformatics tool, revealed that H3K9Me2 candidate genes are enriched in many tightly controlled signaling and cell-type specific pathways. These results demonstrate that monocytes and lymphocytes have distinct epigenomes and H3K9Me2 may play regulatory roles in the transcription of genes indispensable for maintaining immune responses and cell-type specificity.