BACKGROUND Gastrointestinal stromal tumors (GISTs) are rare mesenchymal cancers that affect the gastrointestinal tract and are most often located in the stomach and proximal small intestine. The most common molecular genetic abnormalities underlying GIST carcinogenesis are mutations in the tyrosine kinase gene (KIT) and in the platelet-derived growth factor receptor alpha (PDGFRA) gene. To the best of our knowledge, no cases have been reported so far of synchronous diagnosis of GIST in 2 monozygotic twins presenting with clinical and morphological features of sporadic disease. CASE REPORT This report presents the cases of 2 monozygotic twin sisters who were diagnosed with GIST at the same age and who had different KIT exon 11 tumor mutational statuses. In the current report, the screening examination that led to early detection of GIST in one of the sisters was not motivated by any symptom, but by a GIST diagnosis in her twin a few days before. The literature was reviewed for pathological and molecular features associated with prognosis and treatment response. Furthermore, we identified identical genotypes of KIT and PDGFRA polymorphisms in the DNA of both tumors that might be present in the germline DNA. The present case supports the implementation of specific cancer screening in the context of monozygotic twins, regardless of identification of the genetic components involved. CONCLUSIONS Our report suggests that monozygotic twins with GIST can have different mutational statuses for KIT and PDGFRA. Referral for special screening should be considered for individuals who have a monozygotic twin diagnosed with cancer.