The present invention relates to methods of treating cancer, such as melanoma, by administering a CTLA4 antagonist to a subject with a serum C-Reactive Protein (CRP) concentration that is less than or equal to some amount. The invention further relates to methods of treating cancer by determining the level of serum CRP concentration in a subject, and then administering a CTLA4 antagonist if the CRP concentration is less than or equal to a certain amount. The invention further relates to, among other things, the use of serum CRP concentration as a predictive factor for a subject's response to a cancer treatment.