The question addressed in this study is whether the speed with which a word is recognized depends upon the frequency of related words, and which types of related words have such an influence. For example, does the speed of recognition of the word stair depend at all on the frequency of stairs (an inflectional relative)? Is the speed with which people recognize govern influenced by their knowledge of the word government (a derivational relative)? Is the recognition of fee influenced by the simple fact that the letters overlap with the letters in familiar words such as feet, feed, and feel (nonmorphological relatives)? The authors asked 95 U. S. college students to distinguish stem words from nonwords in a lexical decision task. The stem words were matched for length and individual frequency, but differed substantially in the frequency of their inflectional, derivational, or nonmorphological relatives. The researchers found that the frequency of inflectionally and derivationally related words significantly affected speed and accuracy of recognition of stems; however, these effects were conditioned by the likely age of acquisition for each word, and by the part of speech. Extensive analyses showed that simple letter overlap did not have a significant effect on word recognition. Taken as a whole, the results support the concept of morphologically based word families, that is, the hypothesis that morphological relations between words, derivational as well as inflectional, are represented in the lexicon.