The aim of this study was to examine unique and common causes of problems in reading and arithmetic fluency. 13- to 14-year-old students were placed into one of five groups: reading disabled (RD, n = 16), arithmetic disabled (AD, n = 34), reading and arithmetic disabled (RAD, n = 17), reading, arithmetic, and listening comprehension disabled (TRIPLE, n = 9), and typically developing students (NON-LD, n = 40). Multivariate analyses of covariance and variance component analyses showed that reading problems are characterised by difficulties with phonological processing and with rapid automatic naming. Problems with executive functioning and with digit span were typical for students with arithmetical fluency difficulties. RAD students had problems with phonological processing, rapid naming, executive functioning, and digit span. Impairments in number fact fluency, digit span, loudness perception, speeded sound manipulation, and coding, which all share a fluency component were common to problems with reading and arithmetical fluency.