The immediate, 1 day and 14 days skin reaction was determined in 23 fe
male SLE patients and 23 age-matched controls after standardized expos
ure to ultraviolet light of the UV-A wavelengths (320-440 nm). Eightee
n of the patients and 12 of the controls were photosensitive by histor
y. Eight separate sites on the buttocks were exposed to UV-A light, fo
ur sites at doses between 42 and 252 kJ/m(2) and four sites with longp
ass filters (320, 345, 360 and 375 nm). The reactions on test sites we
re graded by two independent observers unaware of given doses or filte
r 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 reactio
ns were more pronounced to longwave UV-A light (>320 nm) in the patien
ts (P < 0.001). After 14 days, six patients, but no control, had persi
stent erythema (P = 0.04). Interestingly, three of the four patients w
ithout anamnestic photosensitivity did not react on days 1 or 14, whil
e the pattern seen in the controls on day 1 was totally unpredictable
with regard to reported photosensitivity. These findings strongly sugg
est that a considerable proportion of SLE patients show pathological s
kin reactions to physiological doses of longwave UV-A and not only the
far more studied shortwave UV-B (290-320 nm). The responsible chromat
ophore for the UV-A reaction is not known.