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Ground Zero is the sixth episode of the fourth season of ER, and the seventy-sixth episode overall.
Guest Stars: Ellen Crawford (Lydia), Taylor Negron (Mr. Prole), Patrick Kilpatrick (Matthew Lindermulder), Monica Parker (Allergic Woman), Richard Edson (Vinnie LaFontaine), Frances Sternhagen (Millicent Carter), John Aylward (Dr. Donald Anspaugh), Mariska Hargitay (Cynthia Hooper), Paul McCrane (Dr. Robert Romano), Michael Beach (Al Boulet), Clancy Brown (Ellis West), Seth Jaffe (Prole's Assistant), Shaun Toub (Butler), Lily Mariye (Lily), Emily Wagner (Pickman), Monte Russell (Zadro), Benjamin Brown (Cop), Jo Deodato Clark (Margaret Scarletti), James DeBello (Danny Seeff), Josie DiVincenzo (Off. Christine Jeffrey), Benjamin King (Cosi), Dinah Lenney (Shirley), Karl T. Wright (Dr. Eric Norris)
Mark's attitude issues continue to get worse; Carol meets with Carter's grandmother about her funding Carol's idea for a free clinic; Anna is surprised when she finds out that Carter is wealthy; Weaver struggles with budget issues which force her to make a difficult decision while Jeanie's ex-husband, Al tries to convince Jeanie to make a new start outside of Chicago; Doug gets sad news when he hears that his father died in a car accident in California.