OBJECTIVES: The one-stop clinic (OSC) was implemented in 2008 at Gustave Roussy for the diagnosis of patients with thyroid nodules. During a single visit and in a dedicated facility, patients first have a specialised consultation, an echoguided fine needle aspiration cytology (FNAC) with an onsite evaluation and immediate cytopathological diagnosis, and if necessary, a surgical consultation. This service was expected to reduce the number of unnecessary surgeries (by reducing uncertainties linked to suspicious or undetermined lesions at FNAC) then decreasing surgical cost. This study aimed to perform a cost-effectiveness analysis comparing the number of unnecessary surgeries and the cost of one-stop clinic with the strategy previously applied. METHODS: The number of unnecessary surgeries and the diagnosis cost were evaluated prospectively in 605 patients seen at the OSC between 2011 and 2013, and compared to retrospective data of 620 patients who had their FNAC in 2003 or 2004 (Borget, AJCP, 2008). The cost of the diagnostic process was evaluated from the hospital perspective and included the unit cost of FNAC, of unnecessary thyroidectomies (for benign nodules, during 18 months after diagnosis). RESULTS: The implementation of the OSC service was associated with a decrease of undetermined lesions at FNAC (from 25.7 to 1.2%), an increase of suspicious lesions (from 8.7 to 12.5%) and a decrease of unnecessary surgeries (from 19.3% to 13.6%). The cost of diagnosis before and after OSC were estimated at 108 and 126 €, respectively. The extracost of performing a diagnosis by OSC for 605 patients is 21 914€. The OSC implementation then allowed to avoid 38 unnecessary surgeries and an economy of 56 293€. CONCLUSIONS: The extracost of the implementation of the one-stop clinic is largely compensated by the decrease of surgical cost permitted by the reduction in the number of uncertainties.