The Women's 4x100m Medley Relay event at the 10th FINA World Aquatics Championships swam 26 July 2003 in Barcelona, Spain. Preliminary heats swam in the morning session, with the top-8 finishers advancing to swim again in the Final that evening. At the start of the event, the World (WR) and Championship (CR) records were: * WR: 3:58.30 swum by the USA on September 23, 2000 in Sydney, Australia. * CR: 4:01.50 swum by Australia on July 29, 2001 in Fukuoka, Japan