CD34 is a myeloid progenitor cell antigen present in endothelial cells and some other mesenchymal cells, including perivascular and periadnexal dermal fibroblasts. It was evaluated immunohistochemically in uterine stromal tissue and in 4 aggressive angiomyxomas and 6 endometrial stromal sarcomas with potentially related and similar stromal tissues. The stromal cells in normal endocervix and endocervical polyps were strongly CD34 positive irrespective of the cycle phase, and negative for muscle actins. Ectocervical stroma was variably but generally less CD34 reactive. In the endometrium, the CD34 reactivity was limited to the stromal cells of the basal endometrium and was found only in 4 of 20 from proliferative endometria and 1 of 8 from secretory endometria. The uterine cervical and myometrial smooth muscle tissues showed CD34 positive cells only between the muscle bundles and around the vessels. In pelvic aggressive angiomyxomas and endometrial stromal sarcomas the tumour cells were CD34 negative and only the vascular endothelial cells were positive. Endothelial cell-specific antigen, CD31, was identified only in endothelial cells and was not present in the endocervical stroma. These results illustrate the particular immunohistochemical profile of endocervical stromal tissue, namely the strong CD34 expression. The CD34 reactivity of the endocervical tissues should be noted and not confused with neoplasms known to be strongly CD34 positive, such as angiosarcomas, Kaposi's sarcomas and some other spindle cell sarcomas.