The influence of a nocturnal blood pressure dip on cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) has not yet been clarified. We attempted to examine a correlation with the nocturnal blood pressure dip and CADASIL. We monitored circadian blood pressure patterns by the use of a portable blood pressure monitoring device in five patients with CADASIL and 10 age- and sex-matched control subjects. Based on nocturnal fall in mean arterial blood pressure (MABP), we classified patients into extreme dippers (nocturnal reduction of MABP > or =20%), dippers (> or =10% but <20%), nondippers (<10% but > or =0%), and inverted dippers (<0%). Three patients revealed non-dipper and two inverted dipper. Nighttime MABP fall was significantly lower in patients compared with control subjects (P<0.01). This study suggests that a lower nocturnal blood pressure fall may be partly associated with incidence and/or worsening of deep white matter lesions in CADASIL.