The numbers of medical claims of ONFH from 2002 to 2006 were 15,683, 20,746, 22,304, 27,204, and 31,432, respectively. Among the 382 validation samples, 273 (71.5 %) were men and 109 (28.5 %) were women. In total, 209 (54.7 %) patients were hospitalized, while 173 (45.3 %) were treated as outpatients. The number of correct diagnoses among the medical claims was 274 (71.7 %). Significantly lower diagnostic accuracies were found for female patients and nonhospitalized patients (Table 7.2). Overall, 60.3 % of the total claims during 5 years were predicted to be true after regression analysis. The estimated prevalences of true ONFH from 2002 to 2006 constantly increased as 9,870, 12,394, 13,329, 16,230, and 18,691, respectively (Table 7.3). The prevalence per 100,000 population increased during this time period, from 20.53 to 37.96. The average annual prevalence was estimated at 14,103, corresponding to 28.91 per 100,000 population (Table 7.3). The ratio differed from that reported in other studies [19–21]. A male predominance, with a ratio of 3.9:1, was found. Bilateral involvement was found to be relatively low (37.1 %) versus the generally accepted view (50–60 %).