Disregulation of urokinase plasminogen activator (uPA) was assessed in 134 breast cancer specimens. Overexpression of uPA was determined by immunohistochemistry using the specific monoclonal antibody, #394. Gene amplification was assessed by differential polymerase chain reaction, using primers designed to amplify a 111 bp segment of the uPA gene. Overexpression of uPA was detected in 33% of breast cancers, including 4 of 21 in situ carcinomas, 7 of 14 lobular and 2 of 10 tubular carcinomas. Overexpression of uPA did not correlate with the presence or absence of oestrogen receptors. uPA gene amplification was not detected in any cancer. We conclude that uPA gene amplification is not a major mechanism instigating uPA overexpression in breast cancer, and that overexpression is likely to be controlled by other mechanisms.