BACKGROUND AND OBJECTIVES: HTRA1-related cerebral small vessel disease (HRSVD) is a rare hereditary form of cerebral small vessel disease (CSVD) caused by METHODS: This retrospective study included 8 patients with HRSVD, 12 with cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), and 14 with sporadic CSVD (sCSVD). Two neurologists blinded to clinical data counted the number of hypointense dots around the midbrain on SWI. Receiver operating characteristic curve analysis evaluated the optimal threshold of the number that can distinguish HRSVD and CADASIL or sCSVD. In addition, histopathologic analysis including measurement of leptomeningeal vessel diameter and type III collagen deposition was performed on autopsied brains from 3 cases each of HRSVD, CADASIL, and sCSVD and control participants. RESULTS: Patients with HRSVD exhibited a significantly higher number of hypointense dots around the midbrain on SWI compared with CADASIL and sCSVD groups. A threshold of 5 or more dots, termed the "Chocolate Chip Sign," well distinguished HRSVD from CADASIL and sCSVD (area under the curve: 0.817, 95% confidence interval: 0.624-1.00). Three-dimensional SWI reconstruction and 7T MRI confirmed these dots as dilated extraparenchymal vessels. Histopathologic analysis revealed pronounced dilation of leptomeningeal veins with type III collagen accumulation specifically, in HRSVD brains. DISCUSSION: The Chocolate Chip Sign on SWI represents a novel and promising neuroimaging biomarker for HRSVD. This finding holds significant potential for facilitating early diagnosis, prompting timely genetic testing, and appropriate family screening for this rare genetic disorder.