Polymorphous light eruption (PMLE) is another allergic condition related to ultraviolet light alone. It is the most common light-related eruption in North and Central America and Great Britain. The problem may arise at any time of life but most frequently begins sometime before the age of thirty.

The term "polymorphous," which literally means "many forms," refers specifically to the many kinds of skin problems that may be seen in persons with this condition. These include reddish, pimplelike bumps, blisters, eczemalike patches, and large, deep-seated nodules and plaques. In most instances the skin eruptions begin within twenty-four to forty-eight hours of sun exposure and persist for several days. The delayed time frame from exposure to skin rash suggests a delayed hypersensitivity mechanism.

As in the case of solar urticaria, the diagnosis rests heavily on a history of delayed reaction to sun exposure. Phototesting may be helpful in confirming suspected cases.

Fortunately, most people with PMLE have a mild form of the condition and require no therapy at all. Such people typically experience one or two episodes of the problem each year in the early spring and, for reasons that remain obscure, become resistant to the ill effects of the sun for the remainder of the spring and summer seasons. More difficult cases may respond to the use of topical and systemic corticosteroid agents such as prednisone. Certain antimalaria drugs have also proven quite effective. Most recently, the anticancer drug azathioprine (Imuran) was demonstrated to be valuable for treating middle-aged and older patients with the PMLE's eczema variant.