String vessels are collagenous structures connected to capillaries. They have no endothelial cells or lumen. We assessed collagen IV-labeled string vessels in the white matter (WM) of subjects with Alzheimer's disease (AD) (n = 12) and non-AD controls (n = 11) using 100 microm celloidin sections. Ten standard fields were digitally captured and the number and length of normal vessels and string vessels were quantified by computerized image analysis. The WM of the AD-diagnosed individuals contained more strings per mm2 (3.95 +/- 0.49) than comparable WM from controls (1.36 +/- 0.39) (p = 0.0005) and had increased total string vessel length in mm/mm2 (AD = 0.29 +/- 0.04; control = 0.10 +/- 0.03; p = 0.0015). There was a 25% increase (not statistically significant) in vessel density in mm/mm2 in AD subjects (AD = 11.88 +/- 0.87; control = 9.53 +/- 0.78; p = 0.06), presumably due to brain atrophy in the white matter. Although vessel length was slightly increased in AD subjects, they still had more than double the string length per total vessel length (AD = 2.88 +/- 0.38) compared to controls (1.36 +/- 0.27) (p = 0.0057). This increase in string vessels in the white matter of AD subjects suggests a decrease in vascular supply in this disease.