This study aimed to determine the correlation between of ESR, C3, C4, anti-DNA, 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). 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. 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 ESRs, decreasing C3 levels, decreasing C4 levels, elevated anti-dsDNA levels, and increasing SLEDAI-2K scores (all P < 0.01). These findings show that the ESR, C3, C4, and anti-DNA should be used in the evaluation and management of patients with SLE. Also the results show that the BILAG index has construct validity.