PURPOSE: To examine the association between the provision of professional pharmacy services (PPS) and patient complexity as determined by the number of distinct medications dispensed in Ontario. METHODS: We conducted a cross-sectional study among all individuals dispensed one or more medications under the Ontario Public Drug Program (OPDP) between April 1st, 2012 and March 31st, 2013. We compared characteristics of patients receiving 1 or more PPS to those receiving no PPS. To assess the relationship between patient complexity (as measured by the number of chronic medications dispensed) and receipt of PPS, we reported the number and proportion of patients eligible for Ontario Drug Benefits (ODB) who received a PPS within each patient complexity group, and compared these proportions using the Cochran-Armitage test. RESULTS: Over the 1-year study period, 27.1% (N = 799,674 of 2,946,183) of ODB beneficiaries received at least one professional pharmacy service. Among these services, more than two-thirds of the patients received a MedsCheck service (N=511,490; 64.0%). Overall, individuals who received a PPS tended to be older, more likely to reside in a long-term care (LTC) facility, have multiple comorbidities, and were more likely to have been prescribed 9 or more medications in the past year. As patient complexity increased, the proportion of ODB beneficiaries who received PPS also increased; 3.0% of individuals prescribed between 1 and 2 medications in the past year received PPS, while 53.6% of those treated with 13 or more medications received PPS (p<0.0001). CONCLUSIONS: Although the findings of our study suggest the use of PPS increases with patient complexity, many complex patients are not receiving these services. Further studies are required to better understand why patients do not access these services, the impact of professional pharmacy services on patient health outcomes, and their value for the health care system. This article is open to POST-PUBLICATION REVIEW. Registered readers (see “For Readers”) may comment by clicking on ABSTRACT on the issue’s contents page.