10558 Background: Previous studies suggested the role of KIT and PDGFRAmutations on treatment outcome of GIST with IMT, but results are heterogeneous. IHC value of PDGFRA and PDGFRB is not established. Methods: We included patients (pts) treated with IMT as a first line therapy for metastastic or relapsed GIST between 2001 and 2008. Tumor DNA was extracted to investigate the mutation status of KITexon 9, exon 11, PDGFRA exon 12 and 18. IHC stain of c-KIT and PDGFRA/B was performed. We assessed the correlation between the treatment outcome, genetic status and IHC results. Results: A total of 85 pts (M:F=49:36, median age 58.4 years) received IMT 400 mg daily. Location of primary disease included stomach (33), small bowel (34), rectum (10), esophagus (1), and omentum/mesentery (7). Complete and partial responses were achieved in 6% and 62% of pts respectively, while 5% of pts had progressive disease. During median follow up of 28.1 months, estimated median PFS was 39.8 months. KIT exon 11 and 9 mutations were detected in 64% and 5% respectively. Exon 11 mutations included 44 deletions, 2 insertions, 5 substitutions and 3 deletion/insertions. PDGFRA exon 12 and 18 mutations were detected in 2% respectively. Positive rate of c-KIT, PDGFRA and PDGFRB using IHC was 96%, 21%, and 26% respectively. PDGFRA and PDGFRB were co-expressed (p=0.001). PDGFRA mutation did not correlate with PDGFRA/B expression. Clinical response was not different according to the mutation status or IHC expression. PFS of KIT exon 11, KITexon 9, PDGFRA mutants and pts without detectable mutations were not different (p=0.397). Pts with KIT exon 11 balanced mutations (substitution or deletion/insertion) showed longer PFS compared with pts with unbalanced mutations (deletion or insertion) (p=0.014) or pts without exon 11 mutations (p=0.033). Median PFS was shorter in pts lacking c-KIT (p=0.001) expression. PDGFRA/B expression did not influence PFS. Conclusions: Balanced mutation of KIT exon 11 predicted longer PFS, while lack of c-KIT protein expression predicted shorter PFS for GIST pts treated with first line IMT. PDGFRA and B were co-expressed without predictive value. No significant financial relationships to disclose.