The majority of knee arthroplasties (KAs) are performed in patients with osteoarthritis (OA). Although bone mass may be increased in these patients, subjects with knee OA may have an increased risk of hip fracture, possibly due to an increased seveity of falls. However, in patients with KAs, risk of hip fracture has not been studied extensively. We evaluated the association between KAs and hip fracture risk in a population-based case-control study using the Dutch PHARMO Record Linkage System (1991-2002, n = 33,104). Cases were patients with a first admission for hip fracture; controls were matched by age, gender, and geographic location. Neither group had a previous history of fracture. Time since first KA was calculated. Analyses were adjusted for disease and drug history. A 54% increased hip fracture risk was found in patients who underwent KA (adjusted [adj.] OR = 1.54, 95% CI 1.19-2.00). We found a strong effect modification by age in these patients: the youngest patients (aged 18-70 years)were at more increased risk for hip fracture (adj. OR = 2.76, 95% CI 1.16-6.59), while we could not detect a statistical increase in patients aged >80 years. Furthermore, the association tended to be greater during the first few years after surgery, although it did not reach statistical significance. We found that KAs are associated with a 54% increased risk of hip fracture, in particular among adult patients aged <71 years old. Fracture risk assessment could be considered in patients who are about to undergo a KA.