Background and purpose. Mapping of eloquent cortex using blood-oxygen-level-dependent (BOLD) contrast functional MRI (fMRI) has rapidly gained acceptance as part of the evaluation of patients being considered for neurosurgical interventions. The BOLD signal measures local susceptibility in the blood, which can change during periods of increased neuronal activation as a result of alteration in blood flow and cerebral oxygen utilisation. Vascular anomalies could influence the BOLD signal via their effects on both blood flow and susceptibility. Methods. In the present study we have compared the fMRI signal associated with functional activation near arteriovenous malformations and cavernomas in a group of patients referred for pre-surgical mapping of eloquent cortex.Results. The magnitude of the BOLD signal was not different for the cavernoma. group and the AVM group (mean percentage signal change 6.3% vs. 5.5%). For subjects with cavernoma, there was an increase in cavernoma volume on the functional images compared to T1 -weighted anatomical images (mean 570%), and a BOLD signal was only detected outside the enlarged cavernoma.Conclusion. The findings show that susceptibility effects associated with cavernoma, most likely due to hemosiderin deposition, can result in an apparent increase in the separation between the BOLD signal and the cavernoma itself. This could lead to falsely high levels of surgical confidence during neurosurgical resection. Differential patterns of blood flow associated with cavernoma and AVM do not appear to significantly affect the BOLD signal magnitude. @ 2003 Elsevier Ltd. All rights reserved.