Bowel obstruction/perforation reported. Severe (occasionally fatal) infusion-related reactions reported, usually with first infusion. Severe and sometimes fatal mucocutaneous reactions (lichenoid dermatitis, paraneoplastic pemphigus, Stevens-Johnson syndrome, toxic epidermal necrolysis and vesiculobullous dermatitis) reported, occurring from 1-13 weeks following exposure (discontinue in severe mucocutaneous skin reactions). Progressive multifocal leukoencephalopathy due to JC virus infection reported with rituximab use. Rituximab may cause renal toxicity in hematologic malignancies. Tumor lysis syndrome leading to acute renal failure requiring dialysis may occur 12-24 hours following first dose. Rarely, reactivation of hepatitis B (with fulminant hepatitis and hepatic failure) reported with use. Other serious and potentially fatal viral infections, either new or reactivated, associated with use include cytomegalovirus, herpes simplex virus, parvovirus B19, varicella zoster virus, West Nile virus, and hepatitis C. Viral infections may be delayed (occurring up to 1 year after discontinuation of therapy). Use with caution in pre-existing cardiovascular disease, pre-existing pulmonary disease, or prior cardiopulmonary events. Live vaccines should not be given concurrently with rituximab. Increased risk of cardiovascular events.