Background: Although brain metastases (BM) affect 5% of all breast cancer patients and 14% of those with metastatic disease, patients with BM are often excluded from participation in clinical trials. We conducted a structured assessment of the contemporary restrictions to enrolment of, and results for, patients with BM in phase 3 trials published over a period of 23 years in advanced breast cancer. Methods: We used PubMed to search for completed randomized trials published between 01/98 and 12/20. For all eligible trials, two authors independently abstracted data on general characteristics of the studies and detailed information on patient eligibility regarding the presence of BM. Results: We analyzed 210 trials, which enrolled 92,409 eligible patients. Of that total, 162 (77.1%) publications explicitly mentioned eligibility criteria related to the presence of BM and 75 (35.7%) trials reportedly allowed patients with BM, usually with restrictions related to prior brain treatment or stability of lesions. There was a significant increase over time in the percentages of trials allowing patients with BM (p < 0.001), and these trials were more frequently dedicated to HER2-positive or triple-negative disease (p = 0.001). Only 11 trials reported separate results for patients with BM at baseline. The direct treatment activity on BM was usually not reported, although in subgroup analyses the treatment effect in relative terms was usually better among patients with BM than in overall populations. Conclusion: Nearly 36% of phase 3 trials in advanced breast cancer over a 23-year period allowed patients with BM, and this practice is increasing over time. More research is needed to establish the activity of current and promising therapies in patients with BM.