Cyclosporine is the main immunosuppressive drug used for renal transplant reci-pients in order to prevent transplant rejection. Although the drug has increased the survival of patients and grafted organ, it has some side effects independent of its effect on the immune system. This study was done to evaluate the effect of cyclosporine on serum Mg level and its metabolic side effects in renal allograft patients. 157 (62 female and 95 male) renal transplant recipients treated with cyclosporine to prevent transplant rejection were included in the study. Clinical and biochemical data along with cyclosporine levels was documented. Mean serum Mg level was 196 +/- 0.31 mg/dL and mean serum cyclosporine level was 371 +/- 192 microg/dL. Hypomagnesemia was detected in 16 (10.2%) with a negative significant correlation with cyclosporine levels, serum creatinine, plasma LDL, fasting blood sugar and uric acid. In conclusion according to the results of this study there is a significant correlation between cyclosporine and hypomagnesemia. Therefore, routine measurement of serum Mg and its treatment seems necessary to prevent its complications.