The objectives of this study were to (1) evaluate group differences in specific arithmetic competencies among students with various types of learning difficulties (LD) and (2) to examine the influence of attention behaviors on possible group differences. Participants were a community sample of 291 third- and fourth-grade students with reading difficulties (RD) and/or math difficulties (MD), or with no LD (51 MD, 66 RD, 89 MD + RD, and 85 No LD students). Students were administered computerized measures of cognitive addition, subtraction, and estimation (accuracy and response times), as well as other measures. Groups were compared with and without covariation for behavioral inattention and their interactions. Small sums addition, thought to reflect retrieval processes, estimation accuracy, and number knowledge, did not show differences between MD and MD + RD students, although both showed lower performance than the RD and No LD groups. Attention behavior had a variable impact, most typically making group differences larger, but did not alter the general pattern of group differences, except in the case of estimation.