Sunlight is a well-established factor in the induction and exacerbation of lupus erythematosus. Although experimental reproduction of lupus erythematosus lesions with wavelengths shorter than 320 nm was demonstrated previously, the effect of wavelengths longer than 320 nm was not investigated adequately. In this study we show that the action spectrum of lupus erythematosus reaches into the UVA region. A total of 128 patients with lupus erythematosus underwent phototesting with the use of polychromatic UVB and long-wave UVA. Subsets of the disease consisted of discoid lupus erythematosus (n = 86), subacute cutaneous lupus erythematosus (n = 22), and systemic lupus erythematosus (n = 20). Skin lesions clinically and histologically compatible with lupus erythematosus were induced in 64% of patients with subacute cutaneous lupus erythematosus, 42% of patients with discoid lupus erythematosus, and 25% of patients with systemic lupus erythematosus. The action spectrum of the induced lesions was within the UVB range in 33% of patients, in the UVA range in 14%, and in the UVB and UVA range in 53%. In positive test reactions patchy dark erythema and urticarial plaques developed within a few days. In some patients typical discoid lesions persisted for months.