The Carabelli cusp is a tubercle or cuspule, or a groove often seen on the palatal surface of the mesiopalatal cusp of maxillary permanent molars and maxillary second deciduous molars. The purpose of this study was to determine the prevalence, degree of expression of the Carabelli structure, and caries prevalence in the Carabelli structure in the permanent first and second maxillary molars in selected Saudi population. Two hundred and seventy six Saudi patients attending the college of dentistry clinics at King Saud University in Riyadh city were selected for examination. The patients' age ranged from 11 to 55 years old. Patients with severely carious, restored or missing upper first or second maxillary molars on any side were excluded. Both the first and the second upper molars were carefully examined for the occurrence of the Carabelli structure and the presence of caries. The prevalence of the Carabelli structure was 57.6% with a bilateralism of 91.2% in the first maxillary molars and 86.4% in the second molars. The Carabelli structure was found to be 29.7% as a groove expression, and 35.5% as a cusp expression. It was concluded that more than half of the Saudi population have a degree of expression of the Carabelli structure and the Carabelli trait is associated with increased caries incidence. Clinical relevance: Prominent pits and grooves can be foci for plaque retention and caries development, a large tubercle can pose some problems with orthodontic bands.