Monosomy 7 and myelodysplastic syndrome (MDS) are two extremely rare medical conditions. Co-occurrence of these conditions is even less common and associated with an increased risk for an aggressive form of leukemia. Due to the rarity of these conditions, there is limited knowledge of morbidity, especially the impact on behavior and cognitive development in children. In this case series we present three siblings with monosomy 7/MDS who underwent neuropsychological assessments. A range of cognitive skills were assessed including intellectual functioning, verbal and nonverbal skills, memory, attention, executive functioning, academics and fine motor skills. In addition, behavioral and emotional symptoms were assessed with parent ratings. Verbal reasoning was found to be preserved, and none of the siblings were not found to demonstrate problems with externalizing behaviors. Despite these commonalities, there was significant variability in cognitive skills among siblings. Variability in presentation was considered with respect to age, intervention services, and treatment factors. Bone marrow transplantation and age at which the sibling received transplant may predict cognitive risk. Female sex may play a protective role against social difficulties associated with Autism Spectrum Disorder. Additional research is needed to further clarify potential contributing factors to cognitive and behavioral problems in order to inform monitoring and intervention guidelines.