Introduction To identify independent predictors of radiographic progression in psoriatic arthritis (PsA) for patients treated with adalimumab or placebo in the Adalimumab Effectiveness in PsA Trial (ADEPT). Methods Univariate analyses and multivariate linear regression analyses assessed risk for radiographic progression (change in modified total Sharp score, ΔmTSS > 0.5) from baseline to week 24 for C-reactive protein (CRP) and other baseline variables, and for 24-week time-averaged CRP (univariate analysis only). Subanalyses determined mean ΔmTSS for CRP subgroups. Analyses were post hoc, with observed data. Results One hundred and forty-four adalimumab-treated patients and 152 placebo-treated patients were assessed. Mean CRP was 64% lower by week 2 with adalimumab and essentially unchanged with placebo. Univariate analyses indicated that elevated CRP at baseline and time-averaged CRP were strongly associated with radiographic progression for placebo-treated patients but not for adalimumab-treated patients. Multivariate analysis confirmed that elevated baseline CRP was the only strong independent risk factor for radiographic progression (for CRP ≥1.0 mg/dl: odds ratio = 3.28, 95% confidence interval = 1.66 to 6.51, P < 0.001). Adalimumab treatment reduced risk of progression approximately fivefold. The difference between mean ΔmTSS for adalimumab versus placebo was greatest for patients with baseline CRP ≥2.0 mg/dl (-0.5 vs. 2.6). Conclusions Systemic inflammation in PsA, as indicated by elevated baseline CRP, was the only strong independent predictor of radiographic progression. This association was observed predominantly for placebo-treated patients. Adalimumab treatment substantially reduced the overall risk of radiographic progression, and provided greatest radiographic benefit for patients with the greatest CRP concentrations at baseline. Trial Registration Trial registration: NCT00195689.