Abstract Purpose: Anticancer dendritic cell (DC) vaccines require the DCs to relocate to lymph nodes (LN) to trigger immune responses. However, these migration rates are typically very poor. Improving the targeting of ex vivo generated DCs to LNs might increase vaccine efficacy and reduce costs. We investigated DC migration in vivo in humans under different conditions. Experimental Design: HLA-A*02:01 patients with melanoma were vaccinated with mature DCs loaded with tyrosinase and gp100 peptides together with keyhole limpet hemocyanin (NCT00243594). For this study, patients received an additional intradermal vaccination with 111In-labeled mature DCs. The injection site was pretreated with nonloaded, activated DCs, TNFα, or Imiquimod; granulocyte macrophage colony-stimulating factor was coinjected or smaller numbers of DCs were injected. Migration was measured by scintigraphy and compared with an intrapatient control vaccination. In an ex vivo tissue model, we measured CCL21-directed migration of 19F-labeled DCs over a period of up to 12 hours using 19F MRI to supplement our patient data. Results: Pretreatment of the injection site induced local inflammatory reactions but did not improve migration rates. Both in vitro and in vivo, reduction of cell numbers to 5 × 106 or less cells per injection improved migration. Furthermore, scintigraphy is insufficient to study migration of such small numbers of 111In-labeled DCs in vivo. Conclusion: Reduction of cell density, not pretreatment of the injection site, is crucial for improved migration of DCs to LNs in vivo. Clin Cancer Res; 19(6); 1525–33. ©2013 AACR.