Polymorphous light eruption (PMLE), or polymorphic light eruption, is a common acquired cutaneous disorder characterized by a pathological response to ultraviolet radiation (UVR). Erythematous papules, vesicles, and plaques (hence the name "polymorphous") develop minutes to hours after exposure to UVR, such as from sunlight or a tanning bed. The eruption is often pruritic, nonscarring, and photodistributed. Attacks most commonly occur during the spring and early summer months, especially after the first sun exposure of the season, and disappear during the winter. Systemic symptoms (such as fever, malaise, headache, myalgias, and arthralgias) are usually absent. While the eruption can last up to several weeks, most cases resolve within several days.

PMLE is most commonly seen in women with lighter skin phototypes aged 20-30 years, but it can occur in all sexes, ages, and ethnicities. Worldwide, prevalence is estimated to be from 10%-20%, with higher rates in higher altitudes and in Western countries. In a 2023 study of 844 patients, a significant proportion of individuals had darker skin phototypes, and further studies that elucidate prevalence in darker skin are needed.

The etiology of PMLE is not well understood. One theory is that UVR modifies antigens in the skin of PMLE patients to make them immunogenic. There are also data to suggest that PMLE involves a type IV delayed-type hypersensitivity reaction. The action spectrum for PMLE is UVA in most cases, but UVB and both UVA and UVB may also incite the response. Recent studies have shown that PMLE is associated with mutations in the NOD2 and TLR5 genes.

Although the condition frequently recurs, the tendency toward the development of PMLE and the severity of the eruption diminish with repeated sunlight exposure. This phenomenon is termed "hardening." As the summer proceeds, it has been observed that the incidence of new eruptions decreases. Prophylactic phototherapy supports this concept of hardening or tolerance.

Despite the variability of presentations, individual patients tend to experience the same clinical manifestations with each episode.