We report on a case with desmoid tumor sucessfully treated with low-dose interferon alfa. A desmoid tumor was diagnosed in August 1998 in the right shoulder area of a 23-year-old woman. Surgery would probably have permanently impaired muscle function in her shoulder and arm. Therefore, interferon alfa treatment (0.9 million units twice daily subcutaneously) was started in November 1998 (time = 0). The tumor volume based on magnetic resonance imaging (MRI) was initially 16 cm3. The size of the tumor decreased gradually during 12 months of treatment, and in November 1999 (time = 12 months) MRI showed no clear tumor demarcation. This treatment modality may be considered as an alternative to mutilating surgery in patients with desmoid tumor.