BACKGROUND & AIMS: Sarcopenia is common in patients with chronic obstructive pulmonary disease (COPD). Serum creatinine/cystatin C (Cr/CysC) ratio has attracted attention as a surrogate marker for sarcopenia but has not been adequately studied in patients with COPD. Thus, the purpose of this study was to investigate the validity of serum Cr/CysC ratio as a predictor of sarcopenia, evaluate a statistical cut-off value, and assess the relationship between Cr/CysC ratio and clinical factors. METHODS: In this prospective observational study, we enrolled 234 male outpatients with COPD. We determined the relevance of serum Cr/CysC ratio as a surrogate maker for sarcopenia by comparing it with various biomarkers and prospectively investigated the relationship of Cr/CysC ratio with the annual exacerbation rate. RESULTS: Serum Cr/CysC was significantly correlated with handgrip strength (r = 0.53, P < 0.01) and muscle mass (r = 0.44, P < 0.01). The area under the curve for sarcopenia was significantly larger for serum Cr/CysC ratio than for other biomarkers (Cr/CysC: 0.87, CysC: 0.63, Cr: 0.61, albumin: 0.57). Multivariate analysis showed no significant difference in the frequency of acute exacerbations between patients in the low- and high-Cr/CysC group, defined by the cutoff value 0.71; however, the frequency of severe acute exacerbations was significantly higher in the low-Cr/CysC group. CONCLUSION: Serum Cr/CysC ratio can be used accurately, inexpensively, and easily to evaluate sarcopenia in male patients with COPD. Our study shows that patients with Cr/CysC below 0.71 have poor physical clinical factors and are at high risk of severe acute COPD exacerbations.