BACKGROUND The prognostic significance of PIK3CA mutations in colorectal cancer (CRC) remains controversial. Recently, an association between programmed death ligand-1 (PD-L1) and PIK3CA mutations has been reported. The study presented here was conducted to investigate the effect of PIK3CA mutations on the prognosis of CRC patients and the association between PIK3CA mutations and PD-L1. METHODS PIK3CA mutations were analyzed by targeted next-generation sequencing using formalin-fixed paraffin-embedded specimens from 224 primary CRC patients. PD-L1 expression was evaluated by immunohistochemical staining. RESULTS PIK3CA mutations and PD-L1 expression were detected in 21.4% and 10.3% of CRC patients, respectively. PIK3CA mutations were significantly correlated with right-side colon cancer (P=0.011) and were correlated inversely with lymph node metastasis (P=0.026), distant metastasis (P=0.047), and high TNM stage (P=0.036). In univariate analysis, PIK3CA mutations were correlated with longer relapse-free survival in CRC patients. PD-L1 expression was correlated significantly with PIK3CA mutations (P<0.001). CONCLUSIONS PIK3CA mutations were associated with favorable prognostic factors, longer relapse-free survival, and expression of PD-L1. Further investigation is needed to identify whether PIK3CA mutations are a good prognostic factor. Additionally, further studies are needed to understand the mechanisms behind the correlation between PIK3CA mutations and PD-L1 expression.