Diagnosing ADHD based primarily on symptom reports assumes that the number/frequency of symptoms is tied closely to the impairment imposed on an individual’s functioning. That presumed linkage encourages diagnosis more by Diagnostic and Statistical Manual of Mental Disorders (4th ed.) style symptom lists than well-defined, psychometrically sound assessments of impairment. The current study correlated measures reflecting each construct in four separate, large-scale ADHD research samples. Average correlation between symptoms and impairment accounted for less than 10% of variance. Symptoms never predicted more than 25% of the variance in impairment. When an ADHD group was formed according to a measure of current symptoms, the sample size shrunk by 77% when a criterion-based measure of impairment was added. The partial unlinking of symptoms and impairment has implications for decisions about the diagnostic process, research criteria for participant inclusion, prevalence estimates, gender ratios, evaluation of treatment effects, service delivery, and many other issues.