OBJECTIVE To analyse quantitatively the effects of clomiphene citrate (CC) on histological characteristics of proliferative endometrium. METHODS Endometrium biopsies were performed on day 12th of both natural and CC cycles in 14 infertile patients. Nuclear size, shapes and DNA contents of epithelial and glandular epithelial cells, stromal cells were measured or calculated after HE and Feulgeu staining by computerized image analysis technique. Results were compared before and after CC treatment. RESULTS The average area, girth and maximal diameter of the nucler of glandular and epithelial cells in CC cycles were significantly smaller than those of natural cycles (P < 0.05), while the shape factor of stromal nuclea in CC cycles was larger than the natural ones (P < 0.05). DNA contents of glandular, epithelial and stromal nuclea in CC cycles were also smaller than those of natural cycles (P < 0.02). The difference of nuclear DNA content between the 2 cycles was positively correlated with the difference of nuclear area of glandular and epithelial cells (P < 0.01). CONCLUSIONS CC treatment inhibited the endometrial mitosis and proliferation. This effects may begin at the stage of DNA synthesis, therfore DNA contents of endometrial glandular and stromal nuclea also decreased. The action of CC may be related to estrogen receptor synthesis.