Background  Plasma YKL-40 is an inflammatory biomarker. No useful biomarker exists in patients with psoriasis or psoriatic arthritis. Objective  To measure YKL-40 and high-sensitivity C-reactive protein (hs-CRP) in patients with psoriasis or psoriatic arthritis before and during treatment. Methods  In 48 patients with psoriasis, we measured YKL-40, hs-CRP and Psoriasis Area and Severity Index (PASI) at inclusion and in a subgroup of 14 patients, we repeated the measurements after four to six weeks of methotrexate treatment. In 42 patients with psoriatic arthritis, we measured YKL-40 and hs-CRP at inclusion and during 48 weeks of adalimumab treatment. The patients with psoriatic arthritis were divided into responders and non-responders. Results  In patients with psoriasis, the baseline median PASI score was 10.8 and baseline YKL-40 was 45 μg/L. Seventeen per cent had elevated plasma YKL-40 compared with healthy subjects. Baseline PASI and YKL-40 were not correlated (rho = 0.14, P = 0.347) and YKL-40 and hs-CRP remained unchanged after treatment. In patients with psoriatic arthritis, the median pretreatment YKL-40 was 112 μg/L and 43% had elevated YKL-40. YKL-40 decreased in 33 patients who responded to adalimumab (from 112 μg/L to 68 at 48 weeks, P = 0.007). Hs-CRP decreased (from 4.65 mg/L to 0.91, P = 0.013) in the responders. In the non-responders (n = 9), YKL-40 and hs-CRP remained unchanged. Conclusions  YKL-40 is elevated in many patients with psoriatic arthritis, but not in patients with psoriasis. YKL-40 decreased in patients with psoriatic arthritis who responded to treatment. YKL-40 may be a useful biomarker to monitor the effect of treatment with tumour necrosis factor-α inhibitors in patients with psoriatic arthritis.