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-H 0 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 H 0 , 0.64 95% CI 0.57–0.70 at H 6 and 0.63 (95% CI; 0.56–0.69) at H 24 , 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.