That's the finding of the National Psoriasis Foundation.
The type of undiagnosed arthritis is called psoriatic.
Symptoms include joint pain, pain that shifts from one joint to the other, joints that feel hot to the touch and very swollen fingers and toes.
Doctors say it's important to diagnose and treat psoriatic arthritis early in order to prevent or slow joint damage.
In this study 44 percent of patients said it took a year or longer for them to be diagnosed.