Various factors impact the outcome of patients with the diagnosis of cancer. Common treatment modalities of cancer, including surgery, radiation therapy and cytostatic agents, all lead to a systemic inflammatory reaction. In particular, this reaction is of physiological importance as it is crucial for patient recovery. However, in some patients, a self-perpetuating inflammatory response develops due to the presence of unfavorable risk factors, several of which are still unknown, but might lead to a worse disease prognosis. Inflammation has been intimately associated with oxidative stress, that is characterized by an imbalance between pro-oxidants, also termed reactive species, and antioxidants. Systemically, oxidative stress can be quantified by measuring thiols (R-SH, sulfhydryl compounds), which are considered to be regulatory nodes within the extracellular antioxidant network. Most importantly, thiol measurements in serum or plasma form a robust and powerful read-out of the in vivo reduction-oxidation (redox) status as thiols are highly redox-active and thus readily oxidized by circulating reactive species. Therefore, systemic quantification of thiols might be a valuable addition to the clinically available diagnostic and prognostic armamentarium as it is able to reliably capture the overall balance between oxidants and the antioxidant capacity of patients. In this review, we summarized the currently available literature on thiol levels as amendable markers for oxidative stress in patients with lung, prostate and colorectal cancer. Total thiols, native (free) thiols and disulfide levels are significantly altered in these patients compared to healthy individuals. In general, these findings indicate that the extracellular antioxidant capacity is severely affected in patients with these types of cancer. Moreover, lower thiol levels are associated with a lowered overall survival. Future research should focus on exploration of the clinical significance of thiols in cancer.