The reading and mathematics growth of 180 children was examined over 4 points, spanning 2nd and 3rd grades. Initially, 4 achievement groups were identified: difficulties in mathematics but not in reading (MD only), difficulties in mathematics as well as in reading (MD–RD), difficulties in reading but not in mathematics (RD only), and normal achievement in mathematics and in reading. When IQ, income, ethnicity, and gender were held constant, the MD-only group grew at a faster rate in mathematics than did the MD–RD group. In reading, the RD-only and MD–RD groups grew at about the same rate. Reading abilities influence children’s growth in mathematics, but mathematics abilities do not influence children’s growth in reading. Increasingly, state and local agencies are using high-stakes assessments to make important educational decisions. Although there are serious consequences for children who fail them, federal law, under the Individuals with Disabilities Education Act, requires that children with learning disabilities be included in the assessments. Typically high-stakes testing in mathematics begins at the end of third grade. However, little is known about the early achievement growth trajectories of children who are behind in this subject area. For example, are the growth rates for children who show mathematics difficulties (MD) in early primary school different from those for children who show no difficulties? Do some children with MD catch up in their achievement levels while others stay behind? We addressed these issues in the present study by examining children’s early achievement in mathematics longitudinally. Children were individually assessed at four time points over a 2-year period, spanning second and third grades. At the start of the study, we identified two groups of children with MD, that is, children with difficulties in mathematics but not in reading (MD only) and children with difficulties in mathematics and difficulties in reading (MD–RD). For contrast, we also identified two groups of children without MD, namely children with difficulties in reading but not in mathematics (RD only) and children with normal achievement in mathematics and in reading (NA). Although many studies define children with MD as a single