Purpose: To report interim cosmetic and toxicity results of a multicenter randomized trial comparing accelerated partial-breast irradiation (APBI) using three-dimensional conformal external beam radiation therapy (3D-CRT) with whole-breast irradiation (WBI). Patients and Methods: Women age > 40 years with invasive or in situ breast cancer ≤ 3 cm were randomly assigned after breast-conserving surgery to 3D-CRT APBI (38.5 Gy in 10 fractions twice daily) or WBI (42.5 Gy in 16 or 50 Gy in 25 daily fractions ± boost irradiation). The primary outcome was ipsilateral breast tumor recurrence (IBTR). Secondary outcomes were cosmesis and toxicity. Adverse cosmesis was defined as a fair or poor global cosmetic score. After a planned interim cosmetic analysis, the data, safety, and monitoring committee recommended release of results. There have been too few IBTR events to trigger an efficacy analysis. Results: Between 2006 and 2011, 2,135 women were randomly assigned to 3D-CRT APBI or WBI. Median follow-up was 36 months. Adverse cosmesis at 3 years was increased among those treated with APBI compared with WBI as assessed by trained nurses (29% v 17%; P < .001), by patients (26% v 18%; P = .0022), and by physicians reviewing digital photographs (35% v 17%; P < .001). Grade 3 toxicities were rare in both treatment arms (1.4% v 0%), but grade 1 and 2 toxicities were increased among those who received APBI compared with WBI (P < .001). Conclusion: 3D-CRT APBI increased rates of adverse cosmesis and late radiation toxicity compared with standard WBI. Clinicians and patients are cautioned against the use of 3D-CRT APBI outside the context of a controlled trial.