OBJECTIVES To examine patterns of augmentation of extended-release (ER) stimulant regimens with immediate-release (IR) stimulants in an adult population, identify the factors associated with these patterns, and quantify those factors' effects on likelihood of augmentation. METHODS Patients 18 years of age or older with prescription claims for at least a 90-day supply from June 2005 to May 2006 for 1 of the following ER stimulants indicated for attention-deficit/hyperactivity disorder (ADHD)—ER dexmethylphenidate (DEX-XR), ER mixed amphetamine salts (MAS-XR), or osmotic-release oral system methylphenidate (OROS® MPH)—and no ADHD medication use in the prior 180 days were selected from a large managed care database. Descriptive statistics were reported for age, gender, health plan type, region, physician specialty, dose level, selected comorbidities, and stimulant cost by augmentation status and by ER stimulant. Chi-square tests were performed to assess differences in categorical variables, and Student t tests were used to detect differences for mean age. A logistic regression model for predicting positive augmentation status was formulated using backward stepwise regression to evaluate the inclusion of potential explanatory variables. RESULTS Of the 3234 adult patients eligible, 13.8% had IR augmentation with a mean incremental IR drug cost of $15.91 per month. Significant covariates for the logistic model were health plan type (P=0.0022), bipolar disorder diagnosis (P=0.0269), ER product (P<0.0001), and dose level (P=0.0345). Likelihood of augmentation was higher with plan type “‘Other” i.e., not HMO, PPO, or POS), a diagnosis of bipolar disorder, use of DEX-XR or MAS-XR, and dose level of “medium.” CONCLUSIONS While this observational study quantified important aspects of the augmentation of ER stimulants with IR stimulants from a claims data perspective, more research is needed to identify the reasons for augmentation in adults with ADHD (e.g., duration of effect, titration) and to examine patterns of medication taking throughout the day.