It is well known that cervical radiculopathy sometimes causes shoulder pain. Hypothesizing that the cause of painful shoulder is related to the cervical spine in the absence of obvious radiculopathy, we measured the anteroposterior diameter of the spinal canal and the range of motion of the cervical spine in patients with painful shoulder on lateral cervical radiographs of the spine. Painful shoulder was diagnosed in 76 patients (24 men and 52 women; mean age 57.6 years). Patients who reported neck pain or numbness of the upper limbs and patients with neurologic abnormalities were excluded from this study. A control group of 54 asymptomatic volunteers (27 men and 27 women; mean age 55.5 years) was formed. The difference in age between the patient group and the control group was not significant. The anteroposterior diameter of the spinal canal at C5 and C6 in the painful-shoulder group (C5: 12.74 mm; C6: 12.76 mm) was significantly narrower than in the control group (C5: 13.60 mm; C6: 13.79 mm). The range of motion was greatest at C4-5 and smallest at C2-3 in both groups; and there was no significant difference in the range of motion between the painful-shoulder group and the control group. When women only or men only were assessed, the results were nearly the same between groups. The cervical spine without obvious radiculopathy appears to be involved in patients with a painful shoulder. We speculate that the shoulder is affected by irritation of a cervical nerve root or referred pain.