It is well known that peptide-producing endocrine tumours cosecrete immunoreactive chromogranin A with their characteristic hormones. Into this study 187 patients with the diagnosis malignant carcinoids or other malignancies were entered. Using chromogranin A at a cut-off level of 30.3 U/ml it was possible to discriminate between patients in remission and patients suffering a relapse with a sensitivity of 91.7% and a specificity of 96.4%, which may be of important diagnostic value. In our study that lasted over one year we could clearly show that the measurement of chromogranin A is impressively superior to 5-hydroxyindoleacetic-acid for detecting a relapse of carcinoids.