Foot involvement is very frequent in patients affected by psoriatic arthritis (PsA). However, evaluation of the painful foot can be problematic, because it is often difficult to distinguish between arthritis, tenosynovitis, and enthesopathy. Plain radiographs can show bone erosion or other features of joint involvement, but give little information about the soft tissues. We therefore studied foot involvement in 31 PsA patients using high resolution sonography, and compared the results with the findings on x-ray and clinical examination. Ultrasound revealed pathological findings in a large proportion of the patients, most of whom exhibited no clinical (pain or swelling) or radiological signs of foot involvement at the time of the study. Our data suggest that involvement of the tendons and entheses may be more frequent in PsA patients than has thus far been supposed, even in cases of not particularly aggressive disease, and that clinical evaluation tends to underestimate these manifestations.