Several formulations of the aromatase inhibitor anastrozole are available in Brazil. We carried out a postmarketing surveillance of the anastrozole (test) formulation in current use at the Brazilian National Cancer Institute (INCA), by comparing anastrozole through plasma concentrations obtained with the test versus the reference formulation. Thirty-three postmenopausal breast cancer patients participated in an open label, bracketed protocol, comprising 3 successive phases of 8-10 days each. The test formulation was used in phases 1 and 3 and the reference formulation in phase 2. Blood samples were collected in the last 2 days of each phase for LC-MS/MS quantification of anastrozole concentration in plasma. Through anastrozole concentrations ranged between 17.4-86.9ng/ml across the three phases, with median (interquartile ranges) values of 43.7 (37.2-52.7), 41.3 (30.9-50.7) and 43.3 (33.9-55.8) ng/ml in phases 1, 2 and 3, respectively. ANOVA detected no statistically significant difference in anastrozole plasma concentrations across the three phases, consistent with switchability between the reference and test formulations. The genetic component (r GC ) of through anastrozole plasma concentrations, estimated using the repeated drug administration procedure across the three phases, was 0.94, suggesting an important component of genetic variability in anastrozole pharmacokinetics.