The Centers for Disease Control & Prevention (CDC) and two health departments are investigating a cluster of peritonitis cases among patients undergoing peritoneal dialysis (PD). Many of the peritonitis events under investigation were caused by Serratia marcescens or other gram-negative pathogens. The CDC is requesting U.S. clinicians to report peritonitis cases in PD patients treated by the same center that meet the following criteria: Two or more patients with peritonitis caused by Serratia spp. at the same center since January 1, 2019, or An increase in peritonitis caused by gram-negative organisms at the center To report cases, email the Dialysis Coalition (DialysisCoalition@cdc.gov) and Dr. Ana Cecilia Bardossy (ABardossy@cdc.gov) using the subject line: “Peritonitis Cases”. Include your name and contact information (with phone number) and your clinic location (city and state) in your email.