In a phase II clinical trial, 59 patients with progressive medullary thyroid cancer were treated with a 24 mg daily dose of the tyrosine kinase inhibitor lenvatinib over a 28-day period. Of these patients, 36% (95% CI 24–49%) reached the primary end point for lenvatinib treatment, defined as an objective response rate assessed by Response Evaluation Criteria In Solid Tumors v1.0 score and an independent imaging review. Overall, the disease control rate was high (80%, 95% CI 67–89%) and the time to response was only 3.5 months (95% CI 1.9–3.7). The median progression-free survival was 9.0 months. The incidence of adverse effects was low; they included diarrhoea, hypertension and decreased appetite, all of which were easily managed with medication or by modifying the dose of lenvatinib.