The identification of autism susceptibility genes has been hampered by phenotypic heterogeneity of autism, among other factors. However, the use of endophenotypes has shown preliminary success in reducing heterogeneity and identifying potential autism-related susceptibility regions. To further explore the utility of using language-related endophenotypes, we performed linkage analysis on multiplex autism families stratified according to delayed expressive speech and also assessed the extent to which parental phenotype information would aid in identifying regions of linkage. A whole genome scan using a multipoint non-parametric linkage approach was performed in 133 families, stratifying the sample by phrase speech delay and word delay (WD). None of the regions reached suggested genome-wide or replication significance thresholds. However, several loci on chromosomes 1, 2, 4, 6, 7, 8, 9, 10, 12, 15, and 19 yielded nominally higher linkage signals in the delayed groups. The results did not support reported linkage findings for loci on chromosomes 7 or 13 that were a result of stratification based on the language delay endophenotype. In addition, inclusion of information on parental history of language delay did not appreciably affect the linkage results. The nominal increase in NPL scores across several regions using language delay endophenotypes for stratification suggests that this strategy may be useful in attenuating heterogeneity. However, the inconsistencies in regions identified across studies highlight the importance of increasing sample sizes to provide adequate power to test replications in independent samples. © 2006 Wiley-Liss, Inc.