Chromogranin A is a well-known protein constituent in granules of neuroendocrine cells. It is also known that plasma levels of chromogranin A increase considerably in patients with neuroendocrine tumours and thus chromogranin A is used as a marker for these tumours. In the present study, we have shown that fragments of chromogranin A are excreted into the urine in some patients with carcinoid tumours. The chromogranin A molecule appeared in the urine N-terminally cleaved at amino acid positions 116 and 210, which are previously reported cleavage sites of the molecule. The fragments identified were mainly of about 35 kDa in size. The unprocessed chromogranin A molecule was not excreted in the urine. Five out of 40 patients excreting the fragments had slight tubular dysfunction in the kidneys. We also showed that these renally excreted split products of chromogranin A were immunogenic and could be used for production of antibodies against chromogranin A. These antibodies were used both for immunocytochemistry and for the development of a specific and sensitive radioimmunoassay for chromogranin A and its fragments. Measurements of plasma chromogranin A by radioimmunoassay appeared to be a better marker for tumour growth than were measurements of chromogranin A in the urine.