We administered platelet-rich plasma（PRP）therapy for injuries at proximal sites of medial ulnar collateral ligaments（UCL）of elbows in 8 baseball pitchers（1 professional, 2 corporation club, 1 college, 3 high school and 1 playing recreationally）resisted against prior conservative therapies administered for 3-7 months with a mean age of 24 years. The UCL injury was classified as grade IIA in one professional pitcher and in other non-professional pitchers as grade IIB based on magnetic resonance findings. Power Doppler analyses showed areas of pathological hypervascularization at proximal sites of the UCLs in all pitchers. The PRP injections were performed 1 to 3 times during 3 weeks. Seven pitchers accompanied with grade IIB injuries returned to their previous sport activities without symptoms and hypervascularization on power Doppler images disappeared. A one pitcher with grade IIA injury had initially begun interval throwing exercises following twice PRP injections, but, due to recurrent elbow pain and worsening joint instability he eventually underwent a reconstruction surgery.