The objective was to identify cases of incomplete systemic lupus erythematosus (SLE) within a defined population in southern Sweden, risk factors for development of complete SLE (> or = 4 classification criteria) and study outcome of the patients. During prospective retrieval of SLE cases within a defined population in southern Sweden, 28 patients (26 women, two men) with incomplete SLE (< 4 ACR criteria) were identified between 1981 and 1992. All patient records were reviewed and clinical and laboratory data were extracted from standardized formats. Organ damage was defined according to the SLICC/ACR damage index. During follow-ups, 16 of 28 patients developed complete SLE (median 13 years; range 10-20 years). The time to develop complete SLE varied between one and ten years with a median time of 5.3 years. Three patients were anti-DNA positive at inclusion; only one of them developed complete SLE. False positive Wasserman reaction and anti-cardiolipin antibodies (aCl) were only found in patients who developed complete SLE (P < 0.04; Fisher exact test). Six patients had malar rash from the start and they all had complete SLE at follow-up (P < 0.04; Fisher exact test). Of eight patients with arthritis, three developed complete SLE. Thrombocytopenia was only found in two patients, both developing complete disease. At follow-up, patients that developed complete SLE had high SLICC damage scores (mean 1.5) compared with patients that remained as incomplete SLE (mean 0.16). In conclusion, in this follow-up study of patients with incomplete SLE 57% developed complete disease after a median time of 5.3 years. Malar rash and aCl were predictors of complete SLE. Individuals that developed complete SLE were more prone to organ damage.