The processing of a written word can be facilitated by the brief prior presentation of an orthographically similar stimulus. However, for adults, this masked form-priming effect does not occur if the target looks like many other words (i.e. if it has many neighbours). The usual interpretation of this result is that the adult word recognition system is tuned to the differing discrimination demands of words: If a word looks like many others, a more precise recognition procedure is required than if it is orthographically distinctive. This theory has developmental implications: Children should show more form priming for high neighbourhood words than adults and should show a gradual reduction in priming as their written vocabularies become larger. Masked form priming was examined in three groups of developing readers, from grades 2, 4 and 6, and a group of adults. In a lexical decision task, the children showed significantly greater priming overall than the adults for high N words. However, the predicted gradual attenuation across age was not found. Supplementary analyses suggested that the adaptation to lexical density may not occur until a somewhat more advanced stage of reading than we had first expected.