BACKGROUND: Knee osteoarthritis causes functional limitation and disability in the elderly. Vitamin D has biological functions on multiple knee joint structures and can play important roles in the progression of knee osteoarthritis. The metabolism of vitamin D is regulated by parathyroid hormone (PTH). OBJECTIVE: The objective was to investigate whether serum concentrations of 25-hydroxyvitamin D [25(OH)D] and PTH, individually and jointly, predict the progression of knee osteoarthritis. METHODS: Serum 25(OH)D and PTH were measured at the 30- or 36-mo visit in 418 participants enrolled in the Osteoarthritis Initiative (OAI) who had ≥1 knee with both symptomatic and radiographic osteoarthritis. Progression of knee osteoarthritis was defined as any increase in the radiographic joint space narrowing (JSN) score between the 24- and 48-mo OAI visits. RESULTS: The mean concentrations of serum 25(OH)D and PTH were 26.2 μg/L and 54.5 pg/mL, respectively. Approximately 16% of the population had serum 25(OH)D < 15 μg/L. Between the baseline and follow-up visits, 14% progressed in JSN score. Participants with low vitamin D [25(OH)D < 15 μg/L] had >2-fold elevated risk of knee osteoarthritis progression compared with those with greater vitamin D concentrations (≥15 μg/L; OR: 2.3; 95% CI: 1.1, 4.5). High serum PTH (≥73 pg/mL) was not associated with a significant increase in JSN score. However, participants with both low vitamin D and high PTH had >3-fold increased risk of progression (OR: 3.2; 95%CI: 1.2, 8.4). CONCLUSION: Our results suggest that individuals deficient in vitamin D have an increased risk of knee osteoarthritis progression.