Introduction: Psoriasis is a chronic inflammatory disorder with significant morbidity and mortality, but a persistent gap appears to exist for the adequate treatment of patients with moderate to severe disease. As the extent of under-treatment is unknown, we attempted to determine overall treatment patterns and estimate under-treatment using a large database. Methods: Data from the US National Health and Wellness Survey was used to estimate the proportion of patients with mild, moderate or severe psoriasis. The proportion with moderate to severe disease was estimated by excluding those with mild disease, and projecting this to the total insured US population, weighted by age and gender. Using US health plan claims data, patient totals by treatment type were determined between October 1, 2007 and September 30, 2012. Patients had to be continuously enrolled in a health plan and be >= 18 years at the end of the analysis window. Psoriasis was confirmed if patients had at least one claim of any type of psoriasis except psoriatic arthropathy (ICD-9 code 696.1). A monthly treatment history, classified by biologic, traditional oral systemic, phototherapy and topical therapy, was recorded for each patient. Results: There were an estimated 1.7 million insured US patients with moderate to severe psoriasis. Of these, 1 million (59%) were not treated for their condition in the preceding year. Among 695,488 patients who were treated for psoriasis in the preceding year, 346,201 were currently receiving treatment and 349,287 had lapsed treatment. Of the patients lapsed and currently treated in this period, the numbers who received each treatment type were 156,409 (biologic), 222,657 (traditional oral systemic), 22,911 (phototherapy), and 293,511 (topical). A limitation of the study was that only insurance claims were analyzed. Conclusion: Moderate to severe psoriasis remains persistently untreated or under-treated. We suggest that potential barriers preventing access to care be explored.