In 54 patients with lupus erythematosus – 6 with systemic, 19 with disseminated discoid, and 29 with localized discoid lupus erythematosus determination of the 3rd and 4th components of complement (C3 and C4) was performed in a prospective study. In relation to 42 healthy persons, the mean value of C3 proved significantly reduced in systemic lupus erythematosus, and the mean value of C4 was significantly reduced in systemic as well as in disseminated discoid lupus erythematosus. When C3 and C4 were related to disease activity, a depression of complement was found parallel to an increase in disease activity in all three forms of lupus erythematosus. Compared with other serological tests, C3 and C4 afforded the best serological evaluation of the disease activity. Alterations in the clinical condition in 8 of the patients with discoid lupus erythematosus were preceded by changes in C4. A single determination of C4 was diagnostic in 8% of the patients with discoid lupus erythematosus.