Serum parathyroid hormone (PTH) levels increase with decreasing kidney function. Two types of assays for the measurement, the intact PTH (iPTH) assay and the whole PTH (wPTH) assay, are routinely used in clinical settings. Although we usually measure serum iPTH levels because there are many evidences on clinical practice, the iPTH assay detect not only PTH (1-84), which is a physiologically active hormone, but also PTH (7-84), which is a PTH fragment. Whereas, the wPTH assay can mainly detect PTH (1-84). Therefore, Japanese Society of Dialysis Therapy guidelines proposed that estimated iPTH is wPTH multiplied by approximately 1.7 in chronic dialysis patients. However, wPTH/iPTH ratio in non-dialysis chronic kidney disease (CKD) patients remains unclear. Therefore, the aim of the present study was to examine the relationship between wPTH/iPTH ratio and kidney function and other factors affecting wPTH/iPTH ratio. We conducted a cross-sectional observational study including 874 CKD patients (CKD stage 1-5; n=773, CKD stage 5D; n=101) who visited our department from December 2014 to December 2015. We measured serum iPTH and wPTH levels using the Elecsys Intact PTH assay and Elecsys PTH (1-84) assay (Roche Diagnostics). Other laboratory data and clinical characteristics were also evaluated and compared among five CKD stage groups (CKD1-2, CKD3, CKD4, CKD5, and CKD5D). We assessed wPTH/iPTH ratio and the correlation of wPTH/iPTH with various clinical factors. Furthermore, we determined the inflection points of these correlation lines using multiple linear regression analysis. Between five CKD stages groups, there were significant differences in age, past history of coronary artery disease, chronic heart failure, use of calcium carbonate and bisphosphonate, wPTH/iPTH ratio, wPTH, iPTH, calcium, phosphate, alkaline phosphatase, hemoglobin, and blood urea nitrogen levels. Diabetes mellitus, medication of vitamin D, total protein, and HbA1c levels were comparable between the groups. Among each CKD stage groups, wPTH/iPTH ratio was 0.85, 0.81, 0.78, 0.59, and 0.69, respectively. wPTH/iPTH ratio was significantly correlated with serum calcium and phosphate levels only in the CKD5 group, and with the GFR and age in the CKD3, CKD4 and CKD5 group. In addition, an inflection point of the correlation lines between eGFR and wPTH/iPTH ratio was 24.1 mL/min/1.73m2. Similarly, an inflection point for serum calcium levels was 20.0 mL/min/1.73m2 and for serum phosphate levels was 18.0 mL/min/1.73m2, respectively. Our study demonstrated that wPTH/iPTH ratio was lowering with decreasing kidney function possibly due to increasing PTH synthesis and metabolization and accumulation of PTH fragments.