Purpose Ultrasound (US) is a key tool in the evaluation of palpable breast masses and helps to refine the likelihood of malignancy and need for additional diagnostic studies. US is available in many low-resource settings, but there is little expertise. We launched a breast US training program for general practitioners (GPs) and nurses at a rural Rwandan district hospital that is a cancer referral facility. We assessed the skills of the GPs and nurses in diagnostic breast US after intensive training. Methods Four breast radiologists from Boston trained five nurses and four GPs in Rwanda over 9 weeks of in-person training and 21 months of weekly remote mentoring using electronic image review and feedback. During the in-person training, trainees and radiologists evaluated patients separately. Remote assessments were based on emailed image sharing. We compared lesions with radiologist and trainee assessments to calculate trainee sensitivity using the radiologist assessments as the gold standard. Results Of 323 breast and axillary lesions assessed by trainees and radiologists, 279 were breast lesions. Of these, 114 (41%) were evaluated by radiologists in person and 165 (59%) through e-mail. Two hundred thirty-seven lesions (85%) were determined to be breast masses by radiologists, 164 of these as solid masses, 25 complex solid/cystic, 15 definite or probable cysts, 31 normal lymph nodes, and two other masses. The sensitivity of trainees’ assessments in identifying solid masses was 90.2% (95% CI, 85.9% to 94.9%) overall. Among trainees who scanned ≥ 10 lesions, mean sensitivity was 90.6% in the first 14 months and 94.0% in the second 9 months, after in-person training ( P = .3, paired t tests). In cases in which radiologists and trainees perceived solid masses (n = 148), trainees’ sensitivity was 81.4% (95% CI, 72.3% to 90.5%) for detecting suspicious masses or probably benign but in need of additional evaluation ( v benign with no additional evaluation needed). Among trainees who scanned ≥ 10 lesions, sensitivity was 79.1% in the first 14 months and 96.2% in the second 9 months ( P = .03, paired t tests). Conclusion Nurses and GPs in a rural sub-Saharan African facility built strong skills in diagnostic US with in-person training and remote electronic mentoring. The sensitivity of assessments for identifying suspicious masses demonstrated significant improvement after sustained mentorship. Assessment of the impact of the training on patient care and outcomes is ongoing. AUTHORS' DISCLOSURES OF POTENTIAL CONFLICTS OF INTEREST The following represents disclosure information provided by authors of this manuscript. All relationships are considered compensated. Relationships are self-held unless noted. I = Immediate Family Member, Inst = My Institution. Relationships may not relate to the subject matter of this manuscript. For more information about ASCO's conflict of interest policy, please refer to www.asco.org/rwc or ascopubs.org/jco/site/ifc . Sughra Raza Honoraria: Fujifilm Medical Services Travel, Accommodations, Expenses: Fujifilm Medical Services