AbstractDyslexia, or specific reading disability, has been shown in many studies to be familial, though no simple mode of inheritance accounts for all pedigrees. It is likely that the difficulties of genetic analysis are due to heterogeneity within the phenotype. We have undertaken a series of studies of normally intelligent children with dyslexia and their families in a search for phenotypic and genetic evidence of heterogeneity. Family histories with specific attention to difficulties in learning to read and spell were taken for 21 families. School test results were available for probands. Spelling was analyzed and anecdotal information assessed to determine whether visual or auditory difficulties predominated. Among the probands, 11 had predominantly visual problems (with preference for orally presented material) and phonetic spelling errors; 7 had predominantly auditory problems (with preference for visually presented materials), mispronunciations, and dysphonetic spelling errors; and 3 had mixed features. Visual and auditory evoked responses were measured on all the probands and some of the family members, in a search for neurophysiological evidence of abnormality or asymmetry of response. No significant differences were found between hemispheres, between those with visual‐ and those with auditorypredominant dyslexia, or between left‐handed and right‐handed individuals. Both anecdotal family histories and spelling performances provide suggestive evidence that dyslexic patients may be delineated into subgroups with predominantly visual or predominantly auditory impairment and that these subtypes may be under separate genetic predisposition. Further neurophysiological investigation will be required to devise ways of identifying cases before learning disability becomes an educational pr