For clinical epidemiologic and proteomic studies, the control of preanalytic variation, including sample processing and storage, is important. We evaluated the stability of progastrin-releasing peptide (ProGRP) as a marker for the quality control of stored serum and plasma samples. The ProGRP from 23 healthy volunteers was measured serially for 8 hours at room temperature, and the results were validated with clinical samples from the biobank. A significant difference in ΔProGRP was also noted between good-quality (time delay <4 hours before storage) and poor-quality (time delay ≥4 hours before storage) specimens (mean ± SD, 0.17 ± 0.08 vs 0.36 ± 0.14; P < .001; Wilcoxon signed-rank test). With a ΔProGRP cutoff of 0.22, the sensitivity and specificity of detection of the poor-quality samples were 85.7% and 75.0%, respectively, in clinical validations. We demonstrated that ΔProGRP could be used as a marker for quality control in sample processing and storage in biobanks.