Mean age was 60.8 yr (33-84); 83% were white, 66% had visceral disease, 71% (n=73) had measurable disease. For LAS vs Fulv, median PFS was 6.04 mos (95% CI, 2.82–8.04) vs 4.04 mos (95% CI, 2.93–6.04), P=0.138 (HR, 0.699 [95% CI, 0.445–1.125]); PFS at 12 mos was 30.7% vs 14.1%; clinical benefit rate was 36.5% vs 21.6%, P=0.12. Objective response rate for LAS vs Fulv was 13.2% vs 2.9%, P=0.12, with 1 complete response (60-week duration) and 4 partial responses (PR) in the LAS arm versus 1 PR in the Fulv arm. PFS was numerically and consistently greater with LAS vs Fulv when visceral metastasis and/or Y537S ESR1 mutation subgroups were analyzed. ELAINE 1 is the first clinical trial comparing LAS with Fulv in ESR1-mutated mBC patients with progression on CDK4/6i and demonstrating activity of a novel SERM in this setting. All clinical outcomes numerically favored LAS vs Fulv in this signal-seeking study.