Invasion of the corpus luteum by macrophages is a characteristic of luteal regression. Monocyte chemotactic protein-1 (MCP-1), a chemokine that recruits macrophages, is expressed in the rat corpus luteum where it increases in amount during luteolysis. In this study we examined the temporal and spatial expression of MCP-1 and changes in macrophage concentration in the human corpus luteum. Corpora lutea (n = 39) were grouped according to menstrual cycle phase and were examined by immunohistochemistry for MCP-1 and macrophages, and by Northern blot for MCP-1 mRNA. We found increasing amounts of macrophages with progressing luteolysis (P < 0.001). Staining for MCP-1 was stronger in the regressing corpora lutea compared with the staining in corpora lutea of early luteal phase (P < 0.05). MCP-1 was more prominent in blood vessel walls surrounding the corpus luteum than in vessels located far from it. The mean MCP-1 mRNA expression in regressing corpora lutea was higher than that observed in corpora lutea of the early and mid-luteal phase (P = 0.003). In conclusion, we found that MCP-1 expression and the number of macrophages increase with regression of the corpus luteum. MCP-1 is mostly expressed in blood vessel walls surrounding the corpus luteum and may play a role in the recruitment of macrophages to the corpus luteum during its regression.