Abstract Aim To evaluate the usefulness of 11C-methionine PET/CT (MET) in the localization of the parathyroid adenomas and to compare the results with those obtained with the conventional technique in double-phase 99mTc-sestamibi scintigraphy (MIBI). We evaluated the optimal timing to acquire MET images. Material and methods A prospective study that included 14 patients (mean age: 65.5±9.7 years) with primary hyperparathyroidism (PH) who underwent surgery was performed. Mean serum iPTH was 215.8±108pg/mL and serum calcium 10.8±0.9mg/dL. MIBI (planar and SPECT) was obtained 10 min and 2–3h after injection of 740MBq (20mCi) of 99mTc-sestamibi. MET was obtained 10 min and 40 min after injection of 740MBq (20mCi) of 11C-methionine. MIBI and MET images were visually evaluated and compared. A score for 10 min and 40 min MET images from 0 (no abnormal uptake) to 3 (intense uptake) was assigned. Results MIBI and MET were positive and concordant in 11/14 patients and in 10 of them the parathyroid adenoma was correctly localized. In 3/14 MIBI was positive and MET negative (MIBI correctly localized the parathyroid adenoma in 2 of them). According to the timing of MET imaging acquisition, the 10 min and 40 min acquisition showed the same score in 10 patients, it was higher at 10 min acquisition in 3 and in 1 the parathyroid adenoma was only detected at 40 min acquisition. Conclusion MIBI remains the technique of choice for the localization of parathyroid adenomas in patients with PH. MET may play a complementary role in selected patients. Delayed acquisition should be included in the MET protocol when the early acquisition is negative.