Objective: To characterise the cholecystokinin (CCK) receptor subtypes in medullary thyroid cancer by measuring the expression of CCK-A and CCK-B/gastrin receptor mRNA. Design: Open study. Setting: Teaching hospital, Sweden. Subjects: 6 patients with medullary thyroid cancer. Intervention: Pentagastrin stimulation test and measurement of calcitonin concentration. Biopsy specimens were analysed using reverse transcription polymerase chain reaction (RT-PCR). Main outcome measure: Presence of CCK-A and CCK-B/gastrin receptors. Results: All 6 patients with medullary thyroid cancer had positive pentagastrin tests preoperatively. CCK-B/gastrin receptors but not CCK-A receptors were detected by RT-PCR in all six biopsy specimens. By contrast, no CCK receptors were found in normal thyroid tissues or in other thyroid tumours (follicular adenoma, papillary carcinoma, or anaplastic carcinoma). Conclusion: The presence of CCK-B/gastrin receptors in medullary thyroid tumours may have important clinical implications.