 This will be a six-week, randomized, double-blind, placebo-controlled, parallel design study of adult ADHD with naltrexone monotherapy. Eligible and consenting subjects will be recruited into the study. The first visit will consist of a meeting with a study clinician who obtains consent, assesses for eligibility, and completes study rating scales. After this evaluation, subjects will complete a neuropsychological assessment and study rating scales two hours; this visit can take place over multiple days, if necessary. Subjects will then be randomized to naltrexone 50 mg once a day with breakfast or placebo at a ratio of 1:1 to be increased, if tolerated, to 100 mg by week 1. 