Background: Hong Kong has a tripartite healthcare system, where western medicine provided in both public and private sectors coexist with Chinese medicine practice. The purpose of this study is to measure fragmentation of ambulatory care experienced by the non-institutionalized population aged 15 and over in such a tripartite system, thus shed light on the ongoing primary care reform. Methods: This is a cross-sectional secondary data analysis using the Thematic Household Survey, which was conducted by the Hong Kong Census and Statistics Department during November 2009 to February 2010 to collect territory-wide health-related information. Among 18,226 individuals with two or more ambulatory visits during the past 12 months before interview, we grouped each visit into one of the three care segments-public western, private western and Chinese medicine. Two individual-level measures were used to quantify longitudinal fragmentation of care across segments over the one-year period: Most Frequent Provider Continuity Index (MFPC) and Fragmentation of Care Index (FCI). Both are analyzed for distribution and subgroup comparison. A Tobit model was used to further examine the determinants of fragmentation. Results: More than a quarter of individuals sought care in two or all three segments, with an average MFPC of 65% and FCI of 0.528. Being older, female, married, unemployed, uninsured, or born in mainland China, with lower education, lower income, higher number of chronic conditions or poorer health were found to have experienced higher fragmentation of care. We also found that, fragmentation of care increased with the total number of ambulatory care visits and it varied significantly depending on what segment the individual chose to visit most frequently-those chose private western clinics had lower FCI, compared with those chose public western or Chinese medicine as the most frequently visited segment. Conclusions: Even measured at healthcare segment level, people