The present case study describes an adult male with clinical signs of mild Down's syndrome (DS), who performed well at school and reached university level. A karyotype was done on a lymphocyte culture and mosaic trisomy 21 was found in 3% of the 437 cells analysed. Eleven signs from Jackson's checklist were found in the clinical evaluation, which along with the analysis of the subject's dermatoglyphic traits, confirmed the DS diagnosis. Cognitive evaluation was done with several psychological tests and the results were within the average range. This rare phenotypic association shows that normal intellectual development may be possible in DS. This finding could be explained by the low trisomic cell frequency, which may have little effect on the critical tissues for intellectual development, and it might also reflect the wide phenotypic variation in mosaic trisomy 21. Other factors, such as strong family support, early and continued intervention programmes for both physical and speech therapy, and a thorough educational process, also provided opportunities for the development of the cognitive potential of the subject.