Mutations in Krev1 interaction trapped gene 1 (KRIT1) cause cerebral cavernous malformation, an autosomal dominant disease featuring malformation of cerebral capillaries resulting in cerebral hemorrhage, strokes, and seizures. The biological functions of KRIT1 are unknown. We have investigated KRIT1 expression in endothelial cells by using specific anti-KRIT1 antibodies. By both microscopy and coimmunoprecipitation, we show that KRIT1 colocalizes with microtubules. In interphase cells, KRIT1 is found along the length of microtubules. During metaphase, KRIT1 is located on spindle pole bodies and the mitotic spindle. During late phases of mitosis, KRIT1 localizes in a pattern indicative of association with microtubule plus ends. In anaphase, the plus ends of the interpolar microtubules show strong KRIT1 staining and, in late telophase, KRIT1 stains the midbody remnant most strongly; this is the site of cytokinesis where plus ends of microtubules from dividing cells overlap. These results establish that KRIT1 is a microtubule-associated protein; its location at plus ends in mitosis suggests a possible role in microtubule targeting. These findings, coupled with evidence of interaction of KRIT1 with Krev1 and integrin cytoplasmic domain-associated protein-1 alpha (ICAP1 alpha), suggest that KRIT1 may help determine endothelial cell shape and function in response to cell-cell and cell-matrix interactions by guiding cytoskeletal structure. We propose that the loss of this targeting function leads to abnormal endothelial tube formation, thereby explaining the mechanism of formation of cerebral cavernous malformation (CCM) lesions.