Background: Anaplastic thyroid cancer (ATC), a rare highly vascularized tumor, has a dismal outcome. We conducted an open-label study of doublet carboplatin/paclitaxel chemotherapy with or without fosbretabulin in patients with ATC. Methods: Patients were randomly assigned in a 2:1 ratio to 6 cycles of paclitaxel 200 mg/m2 followed by carboplatin AUC 6 on day 1 every 3 weeks (CP), or these drugs were given on day 2 after fosbretabulin 60 mg/m 2 (CP/fosbretabulin) on days 1, 8 and 15. After 6 cycles, patients on the fosbretabulin arm without progression could continue to receive fosbretabulin on days 1 and 8 of a 3-week schedule until progression. The primary end point was overall survival (OS). Results: Eighty patients were assigned (planned, 180) when enrollment was stopped due to rarity of disease and very low accrual. Median OS was 5.2 months [95% confidence interval (CI) 3.1, 9.0] for the CP/fosbretabulin arm (n=55; hazard ratio 0.73 [95% CI 0.44, 1.21]) and 4.0 months [95% CI 2.8, 6.2] for the CP arm (n=25; p=0.22 [log rank test]). One-year survival for CP/fosbretabulin versus CP was 26% versus 9%, respectively. There was no significant difference in progression-free survival between the two arms. Grade 1-2 hypertension and grade 3-4 neutropenia were more common with CP/fosbretabulin. There were no significant adverse cardiovascular side effects. Conclusions: Although the study did not meet statistical significance in improvement in OS with the addition of fosbretabulin to carboplatin/paclitaxel, it represents the largest prospective randomized trial ever conducted in ATC. The regimen is well tolerated, with AEs and deaths primarily related to ATC and disease progression.