To determine the outcome and identify risk factors for evolution into systemic lupus erythematosus (SLE) in a population of incomplete lupus erythematosus (ILE) patients, we studied the clinical and serologic manifestations in a cohort of 87 ILE patients. ILE patients had at least one but less than four of the American College of Rheumatology (ACR) classification criteria of SLE and did not present distinctive clinical features or meet classification criteria of other connective tissue diseases. The patients that remained with ILE were compared with patients that evolved into SLE and with a cohort of 94 SLE patients. The mean disease duration and follow up of ILE patients were 4.4 +/- 4.1 and 2.2 +/- 2.4 years respectively. Eight patients evolved into SLE, but none presented major organ damage. At baseline, patients that remained with ILE were less likely to have photosensitivity, elevated anti-dsDNA and decreased C3 complement than patients that evolved into SLE. At the end of the study, malar rash and oral ulcerations were also less frequent in the ILE group. Compared with all SLE cases, ILE patients were less likely to have photosensitivity, malar rash, oral ulcers, Raynaud's phenomenon, arthritis, low C3, low C4, positive anti-dsDNA, anti-Sm, anti-RNP, anti-Ro and anti-La antibodies at baseline. Hazard analyses showed that malar rash, oral ulcers, elevated anti-dsDNA and decreased C4 were associated with SLE occurrence. In conclusion, this study suggests that ILE represents a mild spectrum of lupus in which mucocutaneous and serological abnormalities are associated with progression into SLE.