Background: Among novel hormonal therapeutic targets for post docetaxel mCRPC, Enzalutamide is an oral androgen-receptor blocker disrupting the androgen-receptor nuclear translocation that significantly increases the OS. In this study we evaluated the potential association of both PTH and Ca levels change in pts with mCRPC after the first Enzalutamide administration. Methods: Between 01 and 05/2015, 20 mCRPC pts relapsed after 2-3 prior lines of therapy (docetaxel, cabazitaxel, abiraterone) have been treated with Enzalutamide that was orally administered at 160 mg/day as continuous dosing. Patient characteristics included: median age 67 years (range 50-84), median baseline PSA 120 ng/ml (range 6-1200), median ECOG P S: 1 (range 0-2), Gleason score ≥ 7. In addition 80% of pts had ≥ 2 metastatic sites. Pretreatment baseline and follow up data including measurement of serum Ca and PTH levels (6.5-36.8 pg/ml), ALP, PSA and QoL parameters were evaluated through all lines of therapy. Pts with bone metastasis received zoledronic acid or denosumab with Ca and vitamin D supplementation. Results: In 18/20 pts with bone disease progression we recorded increased PTH levels and, contrarily, decreased Ca levels after 1 month of Enzalutamide despite vit. D and Ca supplementation. PTH levels remained unchanged after 3 months. In 2/20 pts without bone disease progression PTH ranged normal. All pts reported PSA response ≥ 50%, with improved QoL and are still on treatment since Enzalutamide is well tolerated. We did not find PTH change in bone mCRPC pts treated with prior therapy. Conclusions: Our study showed that increase in PTH levels and reduction in Ca levels and increase in PTH levels after 1 month of Enzalutamide treatment is associated with a dramatic reduction of PSA level. These data support a relationship between PTH and Ca changes in pts treated with Enzalutamide and, thus, their changes level may be adopted in clinical practice as surrogate to reflect the drug activity. No studies have evaluated the variations in serum PTH levels following Enzalutamide treatment and whether these early changes relate to the clinical outcome.