We conducted a post hoc analysis of the vandetanib phase III trial involving patients with advanced medullary thyroid cancer (MTC) to assess the efficacy and safety of vandetanib in patients with progressive and symptomatic MTC. The primary objective of the analysis was to determine progression-free survival (PFS) of these patients.Eligible patients from the ZETA trial were divided into 4 disease severity subgroups: progression and symptoms, symptoms only, progression only, and no progression and no symptoms assessed at baseline. PFS, determined from objective tumor measurements performed by the local investigator, overall survival (OS), time to worsening of pain (TWP), and objective response rate (ORR) were evaluated.Of the 331 patients in this trial, 184 had symptomatic and progressive disease at baseline. In this subgroup, results were similar in magnitude to those observed in the overall trial for PFS (hazard ratio [HR], 0.43; 95% CI, 0.28 to 0.64; P < .0001), OS (HR, 1.08; 95% CI, 0.72 to 1.61; P = .71), and TWP (HR, 0.67; 95% CI, 0.43 to 1.04; P = .07), and the observed adverse events were consistent with the known safety profile of vandetanib. In this subgroup, the ORR was 37% in the treatment arm versus 2% in the placebo arm.Vandetanib demonstrated clinical benefit-specifically, increased PFS-in patients with symptomatic and progressive MTC.