Tumoral calcinosis is a rare disorder of unknown etiology, which is characterized by the developement of large calcified masses overlying the large joints in otherwise healthy subjects. If histologic tests are not confirmed, the condition may be treated erroneously as bursitis. This report describes the case of a 10-year-old man with tumoral calcinosis of the knee, a joint very rarely affected by this unusual disorder. Microscopically the tumor consists of a stroma of chronic inflammatory tissue surrounding cystic spaces containing calcium-rich material. We made excision alone, and the patient made an uneventful recovery with no evidence of recurrence up to 13 months postoperatively.