Background: Warm antibody autoimmune hemolytic anemia (wAIHA) is a rare and often serious disease resulting from antibody‐mediated destruction of red blood cells (RBCs). The SYK inhibitor, fostamatinib, markedly improved Hgb levels in patients with wAIHA during a phase 2, open‐label, multicenter study, in which 9 of 21 (43%) met the primary endpoint (Hgb >10 g/dL with an increase of ≥2 g/dL from baseline by Week 24 without rescue therapy or RBC transfusion). Aims: We report the results of the ongoing extension period of the study. Methods: Adults with primary or secondary wAIHA had to have failed ≥1 prior treatment for wAIHA, have hemoglobin (Hgb) <10 g/dL, an IgG‐positive direct antiglobulin test, haptoglobin <10 mg/dL, and lactate dehydrogenase (LDH) >ULN. Fostamatinib 150 mg BID was taken orally with no food restriction (dose reduction permitted based on tolerability). To enter the extension, patients had to meet the primary endpoint OR show a marked improvement in Hgb and had to have tolerated study drug. Results: Nine patients who completed the initial treatment phase entered the extension: 7 patients met the primary endpoint, 1 was a late responder, and 1 had a beneficial trend during the initial treatment phase. For baseline characteristics, see table. Median Hgb levels increased over the study. Five of 9 patients have an ongoing response to fostamatinib as of July 2018, and none have had rescue therapy or an RBC transfusion. Six of 9 patients are still on the study, and 3 discontinued due to lack of response plus AE (1), consent withdrawn (1), and unknown (1). Nine patients had ≥1 AE, 6 with treatment‐related AEs. AEs were mostly mild to moderate, and the most common AEs were diarrhea and upper respiratory tract infection. Three Grade 3 AEs were reported in 2 patients. AEs resulted in dose reduction in 2 patients, dose interruption in 2, and study drug withdrawn in 1. Summary/Conclusion: Fostamatinib markedly improved Hgb levels in 9 of 21 (43%) patients with wAIHA during a phase 2, open‐label, multicenter study. Adverse events (AE) were manageable and consistent with those previously reported for fostamatinib in other conditions. A randomized, placebo‐controlled, Phase 3 trial is underway. image