Background: This study aimed to determine the correlation between of ESR, C3, C4, anti-dsDNA, and lupus activity and also the construct and criterion validity of the British Isles Lupus Assessment Group (BILAG) index for assessing disease activity in systemic lupus erythematosus (SLE). Methods: Patients with SLE were recruited into a cross-sectional study. Data were analyzed for estimating of SLE disease activity [scores on the BILAG index and Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K)]. Overall BILAG scores were determined by the highest score achieved in any of the individual systems in the respective index. Erythrocyte sedimentation rates (ESRs), C3 levels, C4 levels, anti-double-stranded DNA (anti-dsDNA) levels, and SLEDAI-2K scores were used in the analysis of construct validity. Statistical analyses were performed using ordinal logistic regression for construct validity. Finding: Of the 100 patients with SLE, 90% were women. Their mean ± SD age was 31.1 ± 9.8 years. Increasing overall scores on the BILAG index were associated with increasing ESR level, decreasing C3 levels, decreasing C4 levels, and elevated anti-dsDNA levels, and increasing SLEDAI-2K scores (all P < 0.01). Conclusion: These findings showed that the ESR, C3, C4, and anti-dsDNA should be used in the evaluation and management of patients with SLE. Also the results show that the BILAG Index has construct validity. Keywords: Systemic lupus erythematosus, ESR, C3, C4, Anti-dsDNA, BILAG index, The Systemic Lupus Erythematosus Disease Activity Index