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Photosensitivity is one of the major clinical features of Systemic Lupus Erythematosus (SLE), and is considered to be implicated in the disease pathogenesis. We studied seasonal variations of SLE disease activity at latitude 70 degrees North where there is no sunlight in the winter time, in contrast to 24 h daily sunlight in the summer (midnight sun). The associations between the level of plasma melatonin in June and December with disease manifestations were also studied. Twenty-one SLE patients were examined each month for 1 y, and disease activity was assessed by laboratory parameters as well as clinical disease activity parameters SLEDAI and doctor's global assessment. Melatonin levels were quantified by a RIA-assay. There was no significant change of clinical measures or laboratory parameters of disease activity from one month to the next during the one year, except photosensitive rashes. January was the only month without SLE-flares or arthritis, in contrast to rest of the year. The levels of plasma melatonin were highest in December for seven patients and highest in June for one patient (P < 0.005). Plasma melatonin levels did not correlate with measures of clinical disease activity. At a latitude of 70 degrees North there were no major seasonal variations in SLE disease activity during the one year. There was an accumulation of photosensitivity in the summer months, but no indications of worsening of the disease in the winter months. In contrast to the rest of the year, there was no flare in January which had only 5.6 h of sunshine. The level of p-melatonin did not correlate with measures of disease activity.