Background: A novel tretinoin 0.05% lotion formulation has been shown to be efficacious and well-tolerated, and especially effective in adult female acne patients. While it is perhaps counter-intuitive that patients with more severe disease would show clinically significant improvement with topical monotherapy, topical retinoids have been shown to offer realistic treatment options in these patients. Objective: To evaluate the safety and efficacy of once-daily tretinoin 0.05% lotion in adult females with moderate or severe acne. Methods: Post hoc analysis of two multicenter, randomized, double-blind, vehicle-controlled phase 3 studies. Adult females (>=18 years of age) with moderate (N=551) and severe (N=55) acne were randomized (1:1) to receive tretinoin 0.05% lotion or vehicle, once-daily for 12 weeks. Efficacy assessments included changes in baseline inflammatory/noninflammatory lesions, treatment success (at least 2-grade reduction in Evaluator’s Global Severity Score [EGSS] and clear/almost clear) and quality of life (QoL) using the validated Acne-QoL questionnaire. Safety, adverse events (AEs), and cutaneous tolerability were evaluated throughout. Results: At week 12, efficacy in adult females with moderate acne (EGSS=3) treated with tretinoin 0.05% lotion was significantly greater than that reported with vehicle. Mean percent reduction in inflammatory and noninflammatory lesion counts was 58.5% and 55.5% respectively compared with 50.3% and 39.8% with vehicle (P=0.039 and P<0.001). Treatment success was achieved by 25.4% of subjects by week 12, compared with 15.4% with vehicle (P=0.006). Tretinoin 0.05% lotion was numerically more effective in adult females with severe acne (EGSS=4). Mean percent reduction in inflammatory and noninflammatory lesion counts was 59.0% and 58.8% respectively (compared with 53.5% and 45.5% with vehicle), and treatment success was achieved by 17.9% of subjects (compared with 4.5% with vehicle), with 46.6% of subjects achieving at least a 2-grade improvement in EGSS by week 12. Quality of life improvements with tretinoin 0.05% lotion were significant compared with vehicle in adult females with moderate acne (except role-social), but not in severe acne (probably due to the group size). The majority of AEs were mild and transient; more frequently reported in the moderate acne population where application site pain (2.9%), and application site dryness (5.0%) were the most common, compared with one report (4.5%) of application site pain and dryness in the severe acne population. Local cutaneous safety and tolerability assessments were generally mild-to-moderate and improved by week 12. Limitations: The number of severe subjects enrolled in the studies was considerably less than the number of subjects with moderate acne, and the studies were not powered to demonstrate a difference in efficacy based on acne severity. Conclusions: Tretinoin 0.05% lotion was significantly more effective than vehicle in achieving treatment success and reducing inflammatory and noninflammatory lesions in adult females with moderate acne, with notable improvements in treating adult females with severe acne. It was well-tolerated, and all treatment-related AEs were mild or moderate. J Drugs Dermatol. 2019;18(11):1147-1154.