The objective of this study was to determine tendon involvements and enthesal abnormalities in patients with rheumatoid arthritis (RA) using high-resolution ultrasonographic images and to compare the findings with those seen in patients with ankylosing spondylitis (AS) and healthy controls. A total of 24 patients with RA, 18 with AS, and 20 healthy controls matched by age and body mass index (BMI) were included in the study. All of the patients and controls underwent clinical and ultrasonographic examinations of both lower limbs at five enthesal sites (superior and inferior pole of the patella, tibial tuberosity, Achilles tendon, and plantar aponeurosis) and both upper limbs at two tendon sites (tendons of m. biceps brachii and supraspinatus at the shoulder). High-resolution ultrasonographic examinations were performed to detect bursitis, structure thickness, bony erosion, and enthesophyte. An ultrasonographic score of lower limb enthesitis was calculated using the Glasgow Ultrasound Enthesitis Scoring System (GUESS) in all patients. Tendon involvements and enthesal abnormalities were found significantly more often in the RA group than in controls (p<0.05 to <0.001), but were not found to be different from the AS group (p>0.05). On clinical examination 67 of 336 (19.9%) tendon and enthesal sites were abnormal and on ultrasonographic examination 130 of 336 (38.2%) sites were abnormal in RA patients. The most frequently affected enthesal sites in the lower limbs were suprapatellar, infrapatellar, and Achilles tendon in both the RA and AS groups. The tibial tuberosity was less affected in both groups, and involvement of the plantar aponeurosis was not different from the controls. A statistically significant correlation was found between the Ritchie articular index and GUESS (r=0.578, p=0.008). Tendon involvements and enthesal abnormalities in RA patients were found more often than had been estimated. Further studies are required to validate our results.