Nail abnormalities occur frequently in patients with psoriatic arthritis. This study of the finger nails of 46 patients with psoriatic arthritis, 100 nonpsoriatic rheumatism patients, and 100 nonpsoriatic general medical patients was designed to characterise these abnormalities with particular reference to the severity of nail pitting. The results of the study suggest: (1) Onycholysis alone in the absence of previous injury to the affected nail is in favour of a psoriatic origin for the nail dystrophy. (2) Two or all of onycholysis, horizontal ridging, and nail pitting in the same patient are in favour of a psoriatic origin for the nail dystrophy. (3) The presence or absence of nail pitting alone is a poor discriminator between psoriatic and other causes for nail dystrophy. (4) More than 20 finger nails pits per person is suggestive of a psoriatic cause for the dystrophy. (5) More than 60 pits per person is unlikely to be found in the absence of psoriasis.