How models of health care financing and delivery affect patterns of procedure volumes, outcomes, and volume-outcome associations is not known. We compared volume-outcome studies done in Canada, which provides residents with universal, single-payer health care, with those done in the United States, to determine whether there was a difference in the likelihood of finding statistically significant volume-outcome associations. We analyzed 142 articles, most (90.1%) of which were from the United States. The articles described a total of 291 separate analyses. After adjusting for the clustering of multiple analyses in the same study, the likelihood of finding a statistically significant volume-outcome association was substantially lower in Canadian studies as compared with those from the United States (odds ratio 0.24, 95% confidence interval 0.08 to 0.74, p = 0.01). This result persisted after adjustment for the procedure/condition studied, and the number of study subjects. Canadian volume-outcome analyses are less likely to identify statistically significant volume-outcome associations than US studies, possibly because of the smaller size of some Canadian studies. It is also possible that different models of health care financing and delivery affect patterns of procedure volumes and volume-outcome associations. By promoting competition between hospitals and providers, market-based models may exacerbate existing variations in the quality of hospital care.