Abstract: Summary Background Apremilast, a small-molecule inhibitor of phosphodiesterase 4, works intracellularly to modulate proinflammatory and anti-inflammatory mediator production, and doses of 20 mg twice daily have shown efficacy in the treatment of moderate to severe plaque psoriasis in a 12-week phase 2 study. We assessed the clinical efficacy and safety of different doses of apremilast in the treatment of patients with moderate to severe plaque psoriasis. Methods In this phase 2b, multicentre, randomised, placebo-controlled, dose-ranging study, patients (aged ≥18 years) with moderate to severe psoriasis were randomly assigned (in a 1:1:1:1 ratio) to receive oral placebo or apremilast 10, 20, or 30 mg twice daily at 35 US and Canadian sites between Sept 24, 2008, and Oct 21, 2009. At week 16, patients in the placebo group were assigned apremilast 20 or 30 mg twice daily until week 24. Randomisation was generated with a permuted-block randomisation list via interactive voice response system. For the first 16 weeks, treatment assignment was concealed from both investigators and participants. During weeks 16–24, investigators and participants all knew that treatment was active, but the dose was concealed. The primary endpoint was the proportion of patients achieving at least 75% reduction from baseline psoriasis area and severity index (PASI-75) at week 16. Analyses were by intention to treat; missing values were imputed by last-observation-carried-forward. This trial is registered with ClinicalTrials.gov, number NCT00773734. Findings 89 patients were randomly assigned apremilast 10 mg, 87 apremilast 20 mg, and 88 apremilast 30 mg twice daily; 88 were assigned placebo. At week 16, PASI-75 was achieved in five patients (6%) assigned placebo, ten (11%) assigned apremilast 10 mg, 25 (29%) assigned 20 mg, and 36 (41%) assigned 30 mg. Apremilast 10 mg did not differ significantly from placebo in achievement of the endpoint (odds ratio 2·10; 95% CI 0·69–6·42); for both apremilast 20 mg (6·69; 2·43–18·5; p Interpretation Apremilast, given orally at 20 or 30 mg twice daily, seems to be efficacious, safe, and tolerable for patients with moderate to severe plaque psoriasis. Our results support continuing, longer-term studies. Funding Celgene Corporation.