In the present paper, we address brain-behaviour relationships in children with acquired aphasia, by reviewing some recent studies on the effects of focal brain lesions on language development. Timing of the lesion, in terms of its occurrence, before or after the onset of speech and language acquisition, may be a major factor determining language outcome. However, it is still unclear which are the effects of aphasia occurring between 2 and 5 years of age, a time window which is crucial for acquiring and automatizing the basic rules of native language. A comprehensive review of the literature on acquired childhood aphasia precedes the description of long-term follow-up (20 years) of two identical twins, one of whom became aphasic at 3 years and 4 months after infarction of the left sylvian artery. Psycholinguistic analysis and fMRI data show a slow and incomplete recovery from non-fluent aphasia associated to an intra-hemispheric organization of language. These data, which support the potential but also the limits of neural plasticity during language development, are discussed in the light of the literature on the time-course and neural bases of acquired childhood aphasia. (Contains 3 figures and 4 tables.)