This is a randomized (study drug assigned by chance), open-label (all participants will know the identity of the assigned study drug) study divided into three phases, screening, treatment, and follow-up. During screening, potential patients will be assessed for study eligibility after providing signed informed consent. The treatment phase will comprise a series of 28-day cycles with continuous study treatment until breast cancer progression, when an end-of-treatment visit will be completed before the follow-up phase begins. The duration of participation in the study for an individual patient may be up to approximately 7 years, including follow-up evaluations. Patients will be evaluated for the safety and effectiveness of study treatments. During the treatment phase, patients will take the following study drugs by mouth once daily: abiraterone acetate, 1 g/day, as four 250-mg tablets, on an empty stomach, and patients must not eat for at least 1 hour after abiraterone acetate; prednisone (prednisolone when prednisone is not available), 5 mg/day; and exemestane, 25 mg/day, as a single tablet. The treatment phase will consist of a series of 28-day cycles with continuous study treatment until breast cancer progression. At the planned interim analysis, the Data Review Committee has recommended that further randomization to the abiraterone acetate alone group be stopped and that the study is to be continued otherwise.