Although most children acquire language effortlessly, there are some children who find learning language a difficult and arduous task. This group of subjects resembles a natural experiment and has the potential to provide us with insights into the nature of the biological basis of language. The data show that this disorder is associated with certain genetic factors that can lead to neurological abnormalities. Our studies of the linguistic details of language impairment in English, French, Greek and Japanese have led us to conclude that affected individuals cannot construct normal representations for complex words nor can they construct the rules which should operate on these representations. These data are inconsistent with any explanation in terms of auditory or articulatory processing. Therefore, we must conclude that a genetic disorder can impair the ability to build a normal grammar.