This article describes 136 patients with hereditary cerebral haemorrhages; all patients belonged to families (originally) resident in Katwijk (The Netherlands). Cases of hereditary cerebral haemorrhage have also been reported in NW-Iceland, and in the Dutch coastal village of Scheveningen. Katwijk is a Dutch fishing-village, located 20 miles north of Scheveningen. These 136 cases were encompassed in three large pedigrees, and the disorder followed an autosomal dominant mode of inheritance. No connection between the pedigrees from Iceland, Scheveningen and Katwijk has as yet been established. In our series, sclerosis with amyloid deposits could be observed in roughly a quarter of the small arteries and arterioles in the cerebral cortex and the covering arachnoid. The pathological vessels were irregularly distributed in areas and clusters, possibly leading to superficial cerebral infarcts and, secondarily, to haemorrhages. Our findings are identical with those described in patients from Scheveningen, but different from the Icelandic group. In addition to some differences in the age at onset and in the distribution of the angiopathy, it has recently been demonstrated that the amyloid in our patients is constituted by a microprotein which shows a homology to the beta-protein in Alzheimer's disease and Down's syndrome, while the amyloid in Icelandic cases is formed by an aggregation of cystatin C (gamma trace). An unexpected finding in most of our patients is the accumulation of senile plaque-like lesions in the cerebral cortex. We did not observe Alzheimer's fibrillary tangles in any of our cases.