Cysts of probable müllerian origin have recently been recognized in the mediastinum by Hattori (Virchows Arch. 2005;446:82-84; Chest. 2005;128:3388-3390). In a retrospective study, we found 9 such cases, accounting for 5.5% of a series of 163 consecutive mediastinal nonneoplastic cysts operated in our institution. These cysts occurred in 9 women aged 40 to 58 years (mean, 50.6 years). These women often had overweight (n=4) or various gynecologic history (n=5). Cysts were paravertebral (n=8) or prevertebral (n=1). They were initially classified as bronchogenic or unspecified benign serous cysts. Their diameter measured 1.3 to 5 cm. Their thin wall contained smooth muscle. They were lined by a simple cylindrical or cuboidal, nonmucinous, and often ciliated epithelium resembling uterine tubal epithelium. This epithelium expressed cytokeratin 7, epithelial membrane antigen and estrogen and progesterone receptors. It was negative for cytokeratin 5/6. In the same series, there were 66 bronchogenic cysts, 6 being paravertebral. In conclusion, cysts with müllerian differentiation account for a small proportion of mediastinal cysts and have a usual but nonspecific paravertebral location.