This is a randomized, double-blind, placebo-controlled phase 2 study with a secondary crossover phase at the end of the initial trial to ensure all subjects receive one PRP injection. The goal is to identify what proteins change in the blood following repeated intraarticular knee PRP injections in patients with knee osteoarthritis. The objective is to determine the protein changes resulting from a single autologous PRP injection (5 mL) in comparison to a normal saline control. About 60 subjects will take part in this study by two age groups at UCSF into the following arms: Arm A: PRP injection; Arm B: normal saline injection control. The study aims to demonstrate what benefits PRP has on knee osteoarthritis and methods to best achieve biologic effects. Subjects with a diagnosis of knee osteoarthritis ages 18-45 and 46-70 years old presenting to a University Based sports medicine clinic will be screened for potential eligibility. Subjects who meet all qualifying requirements will be recruited from UCSF's orthopedic and primary care clinics. Subjects will be on study for up to 26 weeks Screening: up to 14 days Treatment: injection of PRP or normal saline; subjects can cross over at week 12 to a PRP if originally in the control injection group Follow-up: 2 weeks post-baseline injection, 12 weeks postbaseline, (14 weeks if crossover patient), 26 weeks.