In this study, plasma concentrations of chromogranin A, calcitonin and carcinoembryonic antigen (CEA) were measured in 40 healthy volunteers as well as in 129 patients with recurrences and/or metastases of neuroendocrine tumors and of medullary thyroid carcinomas (MTCs). A double antibody assay was employed using polyclonal rabbit antibodies to a C-terminal fragment of the protein for detection of human chromogranin A. Using ROC analysis, a cutoff at 22 U/l chromogranin A was calculated. In patients with neuroendocrine tumours, much higher serum concentrations of chromogranin A than for patients with MTC (80% vs. 46%) were measured. The following sensitivities were found: chromogranin A; 46%, calcitonin 100%, CEA 52%. Furthermore, the mean values of chromogranin A concentrations correlated with the tumour mass and/or number of metastases in MTC and neuroendocrine tumours. Evaluation of follow-up studies remains to be completed; however, preliminary results showed similarities regarding the behaviour of chromogranin A and calcitonin. Despite the findings of this study, the observations could not confirm chromogranin A as a reliable marker for metastazing or recurrent MTC.