Recent products of piperacillin/tazobactam (PTZ) from the original manufacturer, previously considered a major cause of galactomannan (GM) false-positivity, are reported not to be related to it. However, data regarding generic PTZ are limited and controversial. To evaluate the effect of generic PTZ on GM false-positivity in Korea, we performed a case–control study in adult patients with cancer. A case–control study was designed. Electronic medical records of cancer patients who were admitted and tested for serum GM between March and June 2014 at a tertiary care university hospital were reviewed. During the study period, a single generic PTZ (C manufacturer, Korea) was used. Patients who received PTZ within 24 h prior to serum GM testing were enrolled. Age- and GM test date-matched non-PTZ patients were selected as controls. A total of 110 patients received PTZ within 24 h prior to serum GM testing during the study period. The GM optical density index (ODI) of the PTZ group did not vary significantly from that of the control group ( p  = 0.251). The percentage of false-positive patients in the PTZ group was also similar to that of the control group ( p  = 0.538). There was no statistical relationship between GM ODI titer and time interval from PTZ administration ( p  = 0.095) or cumulative PTZ dose ( p  = 0.416). In a case–control study that evaluated 220 patients, a generic PTZ in Korea was not related to GM false-positivity.