PURPOSE: PIK3CA and PTEN mutations are prevalent in colorectal cancer (CRC), and potential markers of response to MEK inhibitors and anti-EGFR antibody therapy. Relationships between PI3K pathway mutation, clinicopathological characteristics, molecular features and prognosis remain controversial. EXPERIMENTAL DESIGN: 1,093 stage I-IV CRCs were screened for PIK3CA (exons 9, 20), KRAS (codons 12-13), BRAF (codon 600) mutations and microsatellite instability (MSI). PTEN (exons 3-8) and CpG island methylator phenotype (CIMP) status were determined in 744 and 489 cases. PIK3CA data was integrated with 17 previous reports (n=5,590). RESULTS: PIK3CA and PTEN mutations were identified in 11.9% and 5.8% of CRCs. PTEN mutation was associated with proximal tumors, mucinous histology, MSI-high, CIMP-high and BRAF mutation (P<0.02). PIK3CA mutation was related to older age, proximal tumors, mucinous histology and KRAS mutation (P<0.04). In integrated cohort analysis, PIK3CA exon 9 and 20 mutations were overrepresented in proximal, CIMP-low and KRAS mutated cancers (P=0.011). Comparing PIK3CA exonic mutants, exon 20 mutation was associated with MSI-high, CIMP-high and BRAF mutation, and exon 9 mutation was associated with KRAS mutation (P=0.027). Disease-free survival for stage II/III CRCs did not differ by PI3K pathway status. CONCLUSIONS: PI3K pathway mutation is prominent in proximal colon cancers, with PIK3CA exon 20 and PTEN mutations associated with features of the sessile serrated pathway (MSI-high/CIMP-high/BRAF(mut)), and PIK3CA exon 9 (and to a lesser extent exon 20) mutation associated with features of the traditional serrated pathway (CIMP-low/KRAS(mut)) of tumorigenesis. Our data highlight the PI3K pathway as a therapeutic target in distinct CRC subtypes.