Psoriatic arthritis is a rheumatoid-like joint disease associated with psoriasis of the skin and nails. Although the arthritis component of this syndrome may be clinically indistinguishable from rheumatoid arthritis, the rheumatoid nodules are absent, and serologic tests for rheumatoid factor are negative. Some patients develop a more asymmetrical oligoarthritis affecting large or small joints. Psoriatic arthritis is usually mild, with intermittent flare-ups; however, in rare cases it may progress to crippling arthritis mutilans. This disease affects men and women equally; onset usually occurs between ages 30 and 35.