Disclosed are methods of diagnosing a subject as having a solid tumor or a predisposition to developing a solid tumor. The methods include detecting or quantitating the amount of sCD27 in a serum sample obtained from a subject and comparing that amount of sCD27 with a control value indicative of the basal level of sCD27 present in the serum of a subject that does not have a solid tumor or a predisposition to developing a solid tumor, wherein a reduction of the amount of sCD27 relative to the control value indicates that the subject has the solid tumor or the predisposition to developing the solid tumor. The disclosed methods can be used to monitoring disease progression in a subject or determine a subject's suitability for immunotherapy. Also disclosed are methods of stimulating a subject's immune system by administering a therapeutically effective amount of sCD27 or a functional fragment.