The authors' objective in this article was to explore the accuracy of mothers' estimates concerning their children's developmental functioning, especially with respect to vocabulary and gross motor development, by comparing the results of diagnostic tests administered to both the children and their mothers. The authors studied 55 children with disorders of language development (LD) between the ages of 3 and 6 years using several diagnostic scales concerning child development. The authors assessed the mothers' estimates regarding vocabulary and gross motor development by the same scales as those used for their children. These scales were presented as questionnaires and scored like the tests used for the children. There was a significant tendency toward a general overestimation of a child's developmental functioning regarding vocabulary and gross motor skills. Moreover, the accuracy of the mothers' estimates did not seem to be associated with several selected variables. The results did not correspond to those of other studies that have shown that mothers' estimates provide a good indication for the developmental status of a child--at least for the child's developmental status considering vocabulary and gross motor skills in children with LD. The results support the objection regarding the use of maternal estimates as the only source of information concerning the development of a child among scientific studies, especially if they deal with research on the development of vocabulary or gross motor skills. (Contains 3 tables and 2 figures.)