Summary Long-wavelength ultraviolet A (340–400 nm; UVA1) therapy is currently available in only a few dermatology departments. Equipment capable of delivering this waveband has been available since 1981, but it is only over the past decade that increasing numbers of studies assessing the potential of this as a therapy have been published. High-dose UVA1, which requires expensive and space-occupying apparatus, is effective as a monotherapy for acute flares of atopic dermatitis, but it has not yet been formally assessed as an adjunct, rather than as an alternative to conventional therapies including potent and very potent topical corticosteroids. Low-dose (which can be administered using a standard phototherapy cubicle fitted with appropriate lamps) and medium-dose UVA1 may be less effective for this indication. Another condition for which UVA1 is effective, and is particularly promising because we have no reliably effective treatment already, is localized scleroderma. It also appears to be effective in systemic lupus erythematosus (although it is not yet clear when it is indicated, and its safety needs to be assessed in more patients) and in polymorphic light eruption (although there have been no studies suggesting that UVA1 will have any advantages over standard prophylactic phototherapies). Open studies and case series suggest that UVA1 may prove beneficial for various other diseases, including cutaneous T-cell lymphoma, lichen sclerosus, keloids, systemic sclerosis and hand dermatitis. In the centres where it is available, UVA1 has already proved a useful addition to the range of phototherapies previously available. However, much more research is needed to confirm its efficacy for many of its potential indications, and to determine when and how it should be used.