Objectives: To compare levels of complement components between patients with SLE and patients with SLE and secondary antiphospholipid syndrome and investigate their relation with levels of anti-dsDNA and anticardiolipin antibodies. Materials and methods: A cross-sectional study was conducted at the Department of Rheumatology and clinical Immunology of the University Hospital Split (KBC Split). Included were 74, 45 with SLE and 29 with SLE and secondary APS. Patient data were obtained from the database at the Department for Rheumatology and Immunology as well as at the Institute for Laboratory Diagnostics, KBC Split. The investigated laboratory parameters are: antinuclear antibodies, anti-ds DNA antibodies, C3, C4, IgG, IgM, IgA, Lupus Anticoagulant, IgG and IgM anticardiolipin antibodies, IgG and IgM beta2-gylcoprotein 1 antibodies. Results: All patients with SLE had decreased C3 values, while the C4 values were within the reference interval. Patients with SLE and secondary APS had C3 and C4 levels within the reference interval but at the lower limit. In both groups, aCL IgG and IgM antibodies were mostly negative, whereas anti-ds DNA antibodies were mostly positive. Anti-ds DNA was statistically significantly correlated with C3 (P <0.001) and C4 (P <0.002) in SLE patients without APS, the same correlation was confirmed in patients with SLE and secondary APS for C4 complement levels (P = 0.014). Only the group of patients with SLE without secondary APS show a correlation of C3 and C4 with a positive titer of IgM aCL antibodies (C3: P = 0.036; C4: P = 0.004) as well as C4 with IgG aCL antibodies (P = 0.004). There is no statistically significant difference in complement component levels of C3 and C4 between patients diagnosed with SLE without APS and patients diagnosed with SLE and secondary APS (C3: P = 0.686; C4: P = 0.371). Conclusion: In SLE patients, lowered levels of C3 were confirmed as well as the correlation of reduced complement levels with the specific autoantibody dsDNA. The expected difference between C3 and C4 complement levels was not confirmed between patients with SLE without APS and patients with SLE and secondary APS. Interesting are the correlations of complement and anticardiolipin antibodies, in particular IgM, that suggest a complex role of complement in autoimmune events and the need for further research.