Little is known about the mathematical development of students with intellectual disabilities (ID) in inclusive classrooms. It is important to have a research-based understanding of the subject since inclusive education is becoming the norm in many countries, and an increasing number of students with an ID now attend mainstream schools. We investigated the learning gains of 38 students with ID from 31 grades 2 and 3 inclusive classrooms. Data on mathematics achievement were collected at the beginning and at the end of one school year. A cluster analysis revealed four homogeneous groups that differed significantly in their mathematical progress. Students in the same cluster improved in the same subskills. Prior knowledge is a significant predictor for progress and explains more variance than IQ. In addition, the acquisition of the quantity-number concept, especially the linkage of quantities and numbers, seems to be an important factor for mathematical development. These results show that mathematics instruction needs to be tailored to the specific knowledge profiles of students.