Lymphocyte capping with concanavalin A was studied in 11 patients with hereditary cerebral haemorrhage with amyloidosis (Dutch type) and 10 controls. No difference in capping was found between patients and controls. Abnormal lymphocyte concanavalin A capping has been reported in patients with the Icelandic type of cerebral amyloidosis and in patients with Alzheimer's disease, a disease in which cerebral amyloid angiopathy can also be found. The results suggest a difference in pathogenesis between the Dutch type of cerebral amyloidosis and the other amyloid diseases.