['This study was conducted to find out the optimal vitamin D cutoff point in predicting activity of RA disease.', 'One hundred and two rheumatoid arthritis Saudi patients of both genders were recruited in this study. Vitamin D as 25-hydroxy-vitamin D [25(OH)D] was measured and serum level less than 20 ng/mL defined as deficient patient. Disease activity was measured based on the disease activity score index of a 28-joint count (DAS28) using serum erythrocyte sedimentation rate levels. Receiver operating characteristic (ROC) curves were used to determine the optimal vitamin D cutoff points for identifying disease activity.', 'It has been observed that vitamin D levels were lower (P < 0.05) in patients with high disease activity. A significant inverse correlation between serum 25(OH)D levels and DAS28 (r = -0.277, P = 0.014) was shown. ROC curves results showed that vitamin D less than 12.3 ng/mL predicted high disease activity, and vitamin D more than 17.9 ng/mL predicted low disease activity, with good sensitivity and accuracy results regarding vitamin D.'] Does vitamin D be a Good Marker for Disease Activity of Rheumatoid Arthritis Disease?