Our objective was to characterize clinical features, laboratory findings, concomitant autoimmune diseases, and smoking habits of lupus erythematosus subgroups in genetically homogeneous patients from two Dermatology Departments of Finnish University hospitals. One hundred and seventy eight discoid lupus erythematosus, 55 subacute cutaneous lupus erythematosus, and 77 systemic lupus erythematosus patients were enrolled using patients' charts from institutional database (1995-2006) and during routine control visits. Clustering analysis was performed to reveal natural groupings. Smoking at the onset of disease was significantly more common in all subgroups (57% for discoid lupus erythematosus, 35% for subacute cutaneous lupus erythematosus, and 34% for systemic lupus erythematosus) compared with the age/gender-matched prevalence in the Finnish population, suggesting smoking to be a trigger factor for cutaneous lupus. Leukopenia (38%) and lymphopenia (52%) were observed more often in patients with systemic lupus erythematosus than reported previously. Photosensitivity characterized all groups, especially patients with subacute cutaneous lupus erythematosus (87%). Of the autoimmune diseases, Sjögren's syndrome was the most common (22% of patients with systemic lupus erythematosus), followed by autoimmune thyroid disease (13% of patients with subacute cutaneous lupus erythematosus). The clustering analysis showed environmental factors (smoking) to be more involved in disease development in discoid lupus erythematosus, whereas immunological factors were more significant in initiating systemic lupus erythematosus. The high prevalence of autoimmune thyroid disease, together with photosensitivity, and the clustering profiles suggest that lupus erythematosus subtypes, especially discoid lupus erythematosus, are heterogeneic in their pathomechanisms.