OBJECTIVES: To determine dosing patterns of ustekinumab, a biologic treatment for moderate to severe plaque psoriasis, by analyzing prescription fill data and days supplied from Diplomat Specialty Pharmacy (DSP).  Ustekinumab is delivered as a subcutaneous injection at weeks 0, 4, and every 12 weeks thereafter in two weight based doses: 45mg and 90mg.  METHODS: A retrospective analysis was conducted of pharmacy fill data for patients >18 years receiving >2 fills of ustekinumab from DSP between October 2009 to September 2010. Based on prescribing information (PI), the expected time between first and second dose is 28 days +/- a 7-days and the expected time between subsequent doses is 84 days +/- a 14-day window. For patients receiving 1 vial or syringe per fill, dosing was assigned based upon which strength was initially filled. For patients receiving multiple vials or syringes per fill (e.g. 45 mg x 2), dose was based on days between fills. RESULTS: A total of 711 patients met inclusion criteria; 529 (74.4%) received one vial or syringe in the first fill.  A total of 182 patients received ≥2 vials or syringes, 125 (68.7%) conformed to the expected days recommended from the PI. 57 patients (8.0%) had intervals that fell before or after the expected time. Of the 654 (529 + 125) patients for whom dosing patterns could accurately be assessed, 67.1% were dosed with 45mg. CONCLUSIONS: Over 90% of patients had prescription fill data and intervals that allowed for dosing patterns to be determined; the majority were dosed with 45mg. Some patients received multiple 45mg vials or syringes in one shipment, therefore their dosing may be incorrectly interpreted as higher than their actual dose. To appropriately analyze the dosing patterns of ustekinumab, one must account for the number of vials or syringes supplied per fill, the strength, and the days between fills.