A clinical followup study was performed of joint side tears of the rotator cuff to define the pathogenic mechanisms and factors that affect their evolution. From 1982 to 1992, the authors treated 61 joint side tears, diagnosed by arthrography. Among them, 40 tears were follow-up during conservative treatment. Arthrographies were repeated at a mean of 412 days. There were 16 male and 20 female patients, with a mean age of 61 years. Two male and two female patients had bilateral tears. The average shoulder assessment score according to the Japanese Orthopaedic Association rating at the initial visit to the hospital was 68 points, and 80 points at followup time. Followup arthrographies revealed disappearance of the torn portion in four instances, reduction of the tear size in four, enlargement of the tear size in 21, and progress to full thickness cuff tear in 11 patients. Prognosis of the joint side tear appears to be worse with increasing age, larger tear size, and absence of history of trauma.