In the last years preclinical studies have paved the way for the use of adult muscle derived stem cells for reconstruction of the lower urinary tract. Between September 2002 and October 2004, 42 women and 21 men suffering from urinary stress incontinence (age 36–84 years) were recruited and subsequently treated with transurethral ultrasonography-guided injections of autologous myoblasts and fibroblasts obtained from skeletal muscle biopsies. The fibroblasts were injected into the urethral submucosa, while the myoblasts were implanted into the rhabdosphincter. In parallel, 7 men and 21 women (age 39–83 years) also diagnosed with urinary stress incontinence were treated with standard transurethral endoscopic injections of collagen. Patients were randomly assigned to both groups. After a follow-up of 12 months incontinence was cured in 39 women and 11 men after injection of autologous myoblasts and fibroblasts. Mean quality of life score (51.38 preoperatively, 104.06 postoperatively), thickness of urethra and rhabdosphincter (2.103 mm preoperatively, 3.303 mm postoperatively) as well as contractility of the rhabdosphincter (0.56 mm preoperatively, 1.462 mm postoperatively) were improved postoperatively. Only in two patients treated with injections of collagen incontinence was cured. The present clinical results demonstrate that, in contrast to injections of collagen, urinary incontinence can be treated effectively with ultrasonography-guided injections of autologous myo- and fibroblasts.