Although the degradome, which comprises proteolytic fragments of blood proteins, presents a potential source of diagnostic biomarkers, studies on cancer peptide biomarkers have provided inconsistent conclusions. In the present study, we reevaluated the usefulness of serum degradome analyses for searching peptide cancer biomarker candidates. Particular attention was paid to pre-analytical factors influencing the variability of determined peptide levels, including clotting time and control group selection. Studies were conducted on 44 and 86 serum samples collected from cancer patients and healthy individuals, respectively, using liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS)-based analyses. We identified 1373 unique peptides, nearly 40% of which originated from five blood proteins: fibrinogen alpha chain, apolipoprotein A-IV (APOA4), complement C3, apolipoprotein A-I, and alpha-1-antitrypsin. A set of 118 and 88 peptides exhibited highly significant differences (adjusted p-value ≤ 0.01 and fold change ≥ 2) in pair-wise comparisons of control vs. prostate cancer and control vs. colorectal cancer, respectively, with 37 peptides displaying a consistent direction of change for these pair-wise comparisons. The levels of 67 peptides differed significantly in serum samples collected from healthy individuals immediately prior to colonoscopy and those who underwent colonoscopic examination at least four weeks earlier. Of them, 49 peptides originated from APOA4. Whereas earlier studies, including ours, have utilized fragments of fibrinopeptide A (FPA) to distinguish cancer from healthy cases, here we show that their absolute abundance is a sensitive indicator of clotting time. These observations may have implications for future serum peptidome studies since these issues have not previously been recognized.