This invention provides methods for detecting cancers and for evaluating the prognosis of cancer patients. In particular, the methods of this invention utilize YKL-40 as a marker for the presence or absence of a cancer and for the prognosis (e.g. likelihood of recurrence) of a cancer. Elevated levels of YKL-40 are indicative of the presence of a cancer in undiagnosed subjects and indicate likely recurrence of the cancer in subjects diagnosed as having a cancer.