The FANTOM5 consortium used cap analysis of gene expression (CAGE) to analyze the time course of gene expression over development from 11 days postcoitum (dpc) to adult in 16 developing organs and the whole body of the mouse. Every tissue in the body contains a large number of resident macrophages that initially infiltrate the embryo from the yolk sac. These cells contribute to organogenesis, and their functions diversify during development as they acquire tissue-specific adaptations. In each of the FANTOM5 time courses, the expression of known macrophage-specific genes, including CSF1 receptor (Csf1r), epidermal growth factor-like module-containing mucin-like hormone receptor-like 1 (Emr1), and mer receptor tyrosine kinase (Mertk), was readily detectable and increased with time. We reasoned that genes expressed by macrophages would be strongly correlated in their expression with these known markers and might vary between tissues. We used the network analysis tool, Miru, to extract the sets of coexpressed genes from the time course and identified a core set of coexpressed genes attributable to embryonic macrophages, including some, such as dehydrogenase/reductase 3 (Dhrs3), that may have unique functions in development. The FANTOM5 data also detected the appearance of tissue-specific macrophage-expressed genes, such as T cell Ig and mucin domain-containing 4 (Timd4) and V-set and Ig domain-containing 4 (Vsig4) in liver and sialic acid-binding Ig-like lectin 5 (Siglec5) in lung, and confirmed that macrophage content increases with time in each organ as the proliferative phases end, and tissue-specific gene-expression increases. The FANTOM5 data are available on a comprehensive browser (http://fantom.gsc.riken.jp/zenbu/), which provides a resource for the study of macrophage transcriptional regulation and roles in mouse development.