This study investigated EEG coherence differences between two groups of children with attention-deficit/hyperactivity disorder Combined type (AD/HD), with or without comorbid Reading Disabilities (RD), and normal control subjects. Each group consisted of 20 children between the ages of 8 and 12 years, and groups were matched on age and gender. EEG was recorded during an eyes-closed resting condition from 21 monopolar derivations. Wave-shape coherence was calculated for 8 intrahemispheric electrode pairs (4 in each hemisphere), and 8 interhemispheric electrode pairs, within each of the delta, theta, alpha and beta bands. In the intrahemispheric comparisons, the AD/HD groups compared to controls showed across-hemisphere reductions in coherences in the delta band at longer inter-electrode distances. Interhemispheric coherences in the frontal areas were elevated in theta and reduced in alpha; in the temporal area, coherences were reduced in alpha. Compared with children with AD/HD without comorbid RD, intrahemispheric coherences at shorter inter-electrode distances in children with comorbid RD were reduced in the left hemisphere for slow wave activity, particularly delta. Across hemispheres, the comorbid group also showed a reduced level of intrahemispheric coherence at longer inter-electrode distances in alpha. There were no interhemispheric differences associated with RD. The present results indicate that children with AD/HD and comorbid RD show deficits additional to those found in AD/HD patients without comorbid RD. These involve reduced lateralisation and impaired coupling of frontal and occipital brain regions in children with comorbid RD. Results confirm and clarify the additivity of brain dysfunctions in children with comorbid AD/HD and RD, previously reported by EEG power studies. Findings suggest that optimal treatment of these children should recognise the need to specifically address the RD, in addition to employing a medication regime focussed on the AD/HD symptoms.