Background: Measuring primary health care (PHC) performance through hospitalizations for ambulatory care sensitive conditions (ACSCs) remains controversial-recent cross-sectional research claims that its geographic variation associates more with individual socioeconomic position (SEP) and health status than PHC supply. Objectives: To clarify the usage of ACSCs as a PHC performance indicator by quantifying how disease burden, both PHC and hospital supply and spatial access contribute over time to geographic variation in Finland when individual SEP and comorbidities were adjusted for. Methods: The Finnish Care Register for Health Care provided hospitalizations for ACSCs (divided further into subgroups of acute, chronic, and vaccine-preventable causes) in 2011-2017. With 3-level nested multilevel Poisson models-individuals, PHC authorities, and hospital authorities-we estimated the proportion of the variance in ACSCs explained by selected factors at 3 time periods. Results: In age-adjusted and sex-adjusted analysis of total ACSCs the variances between hospital authorities was nearly twice that between PHC authorities. Individual SEP and comorbidities explained 19%-30% of the variance between PHC authorities and 25%-36% between hospital authorities; and area-level disease burden and arrangement and usage of hospital care a further 14%-16% and 32%-33%-evening out the unexplained variances between PHC and hospital authorities. Conclusions: Alongside individual factors, areas' disease burden and factors related to hospital care explained the excess variances in ACSCs captured by hospital authorities. Our consistent findings over time suggest that the local strain on health care and the regional arrangement of hospital services affect ACSCs-necessitating caution when comparing areas' PHC performance through ACSCs.