Patients with systemic lupus erythematosus (SLE) were evaluated using a telephone questionnaire on the activity of various disease manifestations during the seasons of the past year. The results were compared to those of patients with Behcet's disease (BD), using the same questionnaires, and analyzed in relation to the mean temperature, humidity, barometric pressure, and ultraviolet radiation (UVR) in the patient's location, obtained from the official Israeli Meteorological Service. It was found that SLE patients had a tendency towards winter worsening of clinical manifestations, shown as increased incidence of joint pains, weakness, fatigue, Raynaud's phenomenon, and rash, as well as increased number of hospital admissions, sick leaves, and need to raise the dose of medications. The symptoms of patients with BD were not correlated to seasons of the year, except for increased joint pains in autumn and spring. We suggest that UVR accumulation might cause exacerbations in SLE patients several months after prolonged exposure to sunlight in the summer.