The idiopathic photodermatoses are the largest group of photosensitivity disorders and their pathogenesis is mainly based on immunological mechanisms. Polymorphous, also called polymorphic, light eruption is the most common idiopathic photodermatosis. Juvenile spring eruption has distinct, characteristic features but is usually regarded as a mild variant of polymorphous light eruption. Actinic prurigo is a chronic photodermatosis, most prevalent in the native Indian population of North and Latin America, but also occurring in those of western European ancestry, and reported rarely in other populations. There is a strong association with HLA DR4 in most populations in which this has been studied. Idiopathic solar urticaria is rare, but can be incapacitating in severely afflicted patients. As a pathomechanism, immediate-type hypersensitivity against a putative photoproduct in the skin is often assumed. Solar urticaria can rarely be a feature of phototoxicity due to endogenous porphyrins or some exogenous drugs. Hydroa vacciniforme is rare, begins in childhood and mostly resolves in early adulthood. Hydroa vacciniforme-like lymphoma is a recently recognized cutaneous T-cell lymphoma associated with Epstein–Barr virus. Chronic actinic dermatitis was previously considered almost exclusively a disease of old men but it has increasingly been recognized amongst children, in whom it usually arises on a background of atopic eczema.

Skin testing to provoke skin lesions may be very helpful for the diagnosis of photodermatoses. This can be done with a monochromator or other irradiation source, phototesting on small areas of skin or by larger provocation skin testing. Photopatch testing is performed in patients with suspected exogenous chemical photosensitization, such as to sunscreens.

Prophylaxis and treatment of photodermatoses is based on photoprotective measures and skin hardening by narrow-band UVB, rarely other forms of phototherapy or by photochemotherapy. In some instances, topical or systemic antioxidants may be useful. Idiopathic solar urticaria may be treated with plasmapheresis and in actinic prurigo thalidomide has proven effective.