Objective: To determine the etiologic yield in young children with single domain developmental delay (either developmental language disorder or isolated motor delay) after a specialty diagnostic evaluation. Methods: During an 18-month period, all children <5 years of age, who were consecutively referred to pediatric neurology or developmental pediatric clinics at a single tertiary pediatric center, were prospectively enrolled. Etiologic yield was determined after completion of clinical assessments and selected laboratory studies requested by the evaluating physician. Results: Seventy-two children (60 boys) were found to have a developmental language disorder, and 22 children (11 boys) had isolated motor delay, of whom 6 had an associated diagnosis of cerebral palsy. An etiologic diagnosis was rarely made in the children with developmental language disorder (3/72, 4.1%). Laboratory investigations (metabolic, cytogenetic, imaging), aside from audiometry, were uniformly uninformative. In those children with isolated motor delay, an etiology was apparent in more than half (13/22, 59%). Slightly more than half (7/13, 54%) of etiologies identified in this group were potentially preventable. Successful etiologic determination in children with motor delay often had an impact on recurrence risk estimation, medical management, or specific therapy offered (8/13, 62%). The presence of physical findings on initial assessment was found to be highly predictive of successful etiologic determination in children with isolated motor delay (13/17 vs 0/5, P =.002). Conclusion: Etiologic yield differs substantially according to the subgroup of single domain developmental delay. (J Pediatr 2000;137:633-7)