We report on a 30‐year‐old woman with de novo ring chromosome 12 mosaicism, 46,XX,r(12)(p13.3q24.3)/46,XX. In addition to the clinical manifestations generally observed in “ring syndrome” cases such as growth retardation, short stature, microcephaly, and mental deficiency, she had a broad nasal bridge, micrognathia with overbite, under‐developed breasts, mild dorsal scoliosis, clinodactyly of the fifth fingers with a single interdigital crease, symphalangism of thumbs, tapering fingers, mild cutaneous syndactyly between the second and third toes, multiple café‐au‐lait spots, sebaceous acne on the face and back, and mild dystrophic toenails. She developed a large, pedunculated uterine leiomyoma at age 28 years. To our knowledge, uterine leiomyoma in association with r(12) has not been reported previously. However, a gain of chromosome 12 and translocations involving 12q14‐15 have been described. © 1996 Wiley‐Liss, Inc.