Background: To assess the ability of procalcitonin (PCT) to distinguish between bacterial and nonbacterial causes of patients with severe acute exacerbation of COPD (AECOPD) admitted to the ICU, we conducted a retrospective analysis of two prospective studies including 375 patients with severe AECOPD with suspected lower respiratory tract infections. PCT levels were sequentially assessed at the time of inclusion, 6 h after and at day 1, using a sensitive immunoassay. The patients were classified according to the presence of a documented bacterial infection (including bacterial and viral coinfection) (BAC + group), or the absence of a documented bacterial infection (i.e., a documented viral infection alone or absence of a documented pathogen) (BAC- group). The accuracy of PCT levels in predicting bacterial infection (BAC + group) vs no bacterial infection (BAC- group) at different time points was evaluated by receiver operating characteristic (ROC) analysis. Results: Regarding the entire cohort (n = 375), at any time, the PCT levels significantly differed between groups (Kruskal-Wallis test, p < 0.001). A pairwise comparison showed that PCT levels were significantly higher in patients with bacterial infection (n = 94) than in patients without documented pathogens (n = 218) (p < 0.001). No significant difference was observed between patients with bacterial and viral infection (n = 63). For example, the median PCT-H0 levels were 0.64 ng/ml [0.22-0.87] in the bacterial group vs 0.24 ng/ml [0.15-0.37] in the viral group and 0.16 ng/mL [0.11-0.22] in the group without documented pathogens. With a c-index of 0.64 (95% CI; 0.58-0.71) at H0, 0.64 [95% CI 0.57-0.70] at H6 and 0.63 (95% CI; 0.56-0.69) at H24, PCT had a low accuracy for predicting bacterial infection (BAC + group). Conclusion: Despite higher PCT levels in severe AECOPD caused by bacterial infection, PCT had a poor accuracy to distinguish between bacterial and nonbacterial infection. Procalcitonin might not be sufficient as a standalone marker for initiating antibiotic treatment in this setting.