The immediate, 1 day and 14 days skin reaction was determined in 23 female SLE patients and 23 age-matched controls after standardized exposure to ultraviolet light of the UV-A wavelengths (320-440 nm). Eighteen of the patients and 12 of the controls were photosensitive by history. Eight separate sites on the buttocks were exposed to UV-A light, four sites at doses between 42 and 252 kJ/m2 and four sites with longpass filters (320, 345, 360 and 375 nm). The reactions on test sites were graded by two independent observers unaware of given doses of filter location. All patients and controls reacted with immediate erythema irrespectively of the presence or absence of photosensitivity. After 1 day, 39% of controls and 78% of patients had erythema and the reactions were more pronounced to longwave UV-A light (> 320 nm) in the patients (P < 0.001). After 14 days, six patients, but no control, had persistent erythema (P = 0.04). Interestingly, three of the four patients without anamnestic photosensitivity did not react on days 1 or 14, while the pattern seen in the controls on day 1 was totally unpredictable with regard to reported photosensitivity. These findings strongly suggest that a considerable proportion of SLE patients show pathological skin reactions to physiological doses of longwave UV-A and not only the far more studied shortwave UV-B (290-320 nm). The responsible chromatophore for the UV-A reaction is not known.