Purpose: Advances in personalized medicine have produced novel tests and treatment options for women with breast cancer. Relatively little is known about the process by which such tests are adopted into oncology practice. The objectives of the present study were to understand the experiences of medical oncologists with multigene expression profile (gep) tests, including their adoption into practice in early-stage breast cancer, and the perceptions of the oncologists about the influence of test results on treatment decision-making. Methods: We conducted a qualitative descriptive study involving interviews with medical oncologists from academic and community cancer centres or hospitals in 8 communities in Ontario. A 21-gene breast cancer assay was used as the example of gep testing. Qualitative analytic techniques were used to identify the main themes. Results: Of 28 oncologists who were approached, 21 (75%) participated in the study [median age: 43 years; 12 women (57%)]. Awareness and knowledge of gep testing were derived from several sources: international scientific meetings, participation in clinical studies, discussions with respected colleagues, and manufacturer-sponsored meetings. Oncologists observed that incorporating gep testing into their clinical practice resulted in several changes, including longer consultation times, second visits, and taking steps to minimize treatment delays. Oncologists expressed divergent opinions about the strength of evidence and added value of gep testing in guiding treatment decisions. Conclusions: Incorporation of gep testing into clinical practice in early-stage breast cancer required oncologists to make changes to their usual routines. The opinions of oncologists about the quality of evidence underpinning the test affected how much weight they gave to test results in treatment decision-making.