Medullary thyroid carcinoma (MTC) occurs sporadically but may also be inherited as part of the multiple endocrine neoplasia (MEN) type 2 syndrome. Screening of the patients and first degree relatives annually with basal and provocative tests for serum immunoreactive calcitonin (CT) levels is essential and enables potentially curative disease. Pentagastrin and calcium are the usual provocative agents used worldwide. We used endogenous gastrin (GT) release achieved by omeprazole, 20 mg b.i.d., to stimulate CT in 9 MTC, in 3 MEN 2A family members, and in 50 healthy control subjects. A steady and significant increase both in GT and CT levels was achieved in 9 MTC patients and 3 of the 14 family members tested, whereas in healthy controls the CT increase stimulated by GT was insignificant. Preliminary results showed that this new, safe, cheap, and outpatient-basis test can be used in MTC diagnosis, follow-up, and screening.