INTRODUCTION: Colorectal cancer is the third most common cancer in the United States. The use of bevacizumab (Avastin), a vascular endothelial growth factor (VEGF) inhibitor, has been increasing due to observed improvement in metastatic colon cancer survival, but so has the incidence of bowel perforation. We present one unusual complication of bowel perforation, a colovesical fistula in a colorectal cancer patient treated with bevacizumab.CASE PRESENTATION: A 54-year-old white male diagnosed with Stage IV colorectal cancer was treated with folinic acid, leucovorin, fluorouracil, oxaliplatin (FOLFOX) and bevacizumab. Two months later, he developed pneumaturia and fecaluria. CT showed a rectosigmoid colovesical fistula. A laparoscopic diverting colostomy was created to overcome the proximal retention of feces and the fecaluria.DISCUSSION: Colovesical fistulas more commonly result from diverticulitis, cancer, or Crohn's, but rarely can arise from radiation or chemotherapy. Our patient had two risk factors, colorectal carcinoma and bevacizumab use. Although colon cancer itself can cause a colovesical fistula, at the time of diagnosis, our patient had an intact fat pad between the colon and bladder on CT and did not have symptoms consistent with a fistula, suggesting that bevacizumab was the culprit. The exact mechanism of action leading to bowel perforation is not completely understood. Three theories include first, the idea that bevacizumab increases the risk of thrombosis, second, that tumor destruction creates an area of weakness more prone to perforation, or third, it slows down wound healing, causing leakage at the anastomotic site following surgery.CONCLUSION: Hospitalists encounter patients with colorectal cancer on a regular basis, so clinicians must be aware of the uncommon but potentially serious side effect of bowel perforation when bevacizumab is used. This case has illustrated an even more rare complication, the formation of a colovesical fistula that was treated with laparoscopic surgical intervention with a diverting colostomy.