Purpose: Estimates of the expected co-occurrence rates of idiopathic language disorder and attention-deficit/hyperactivity disorder (ADHD) provide a confusing and inconsistent picture. Potential sources for discrepancies considered so far include measurement and ascertainment biases (Redmond, 2016a, 2016b). In this research symposium forum article, the potential impact of applying different criteria to the observed co-occurrence rate is examined through an appraisal of the literature and an empirical demonstration. Method: Eighty-five cases were selected from the Redmond, Ash, et al. (2019) study sample. Standard scores from clinical measures collected on K-3rd grade students were used to assign language impairment status, nonverbal impairment status, social (pragmatic) communication disorder status, and ADHD status. Criteria extrapolated from the specific language impairment (Stark & Tallal, 1981), developmental language disorder (Bishop et al., 2017), and "Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition" language disorder (American Psychiatric Association, 2013) designations were applied. Results: The "Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition" language disorder designation and its separation of language disorder from the social (pragmatic) communication disorder designation provided the clearest segregation of idiopathic language deficits from elevated ADHD symptoms, showing only a 2% co-occurrence rate. In contrast, applying the broader developmental language disorder designation raised the observed co-occurrence rate to 22.3%. The specific language impairment designation yielded an intermediate value of 16.9%. Conclusions: Co-occurrence rates varied as a function of designation adopted. The presence of pragmatic symptoms exerted a stronger influence on observed co-occurrence rates than low nonverbal abilities. Impacts on clinical management and research priorities are discussed.