The presence of post-tensioning tendons in webs decreases the load-carrying capacity of the compression struts that are necessary to transfer shear forces. This effect has been known for a long time, but was initially not included in design codes. In particular, older bridge designs often included fairly large ducts in rather thin webs. This paper presents the results of an extensive experimental investigation on the effect of the presence of post-tensioning ducts on the shear capacity of thin-webbed post-tensioned girders. The study included a series of large-scale tests performed in the laboratory on post-tensioned girders extracted from an existing bridge.