Vitamin D could have important immunomodulatory effects in psoriasis. To measure 25-hydroxyvitamin D [25(OH)D], parathyroid hormone (PTH) and calcium serum levels in patients with psoriasis and the associations with some relevant clinical features. A cross-sectional study was conducted over 1 year including 145 patients with chronic plaque psoriasis, 112 patients with rheumatoid arthritis (RA) and 141 healthy controls. 25(OH)D, PTH and calcium serum levels were measured in a centralized laboratory. Demography, comorbidities, disease severity and exposure time to sunlight (which was derived by questionnaire) were collected. The prevalence of vitamin D deficiency [25(OH)D levels <20ngmL(-1) ] in patients with psoriasis was 57·8% vs. 37·5% in patients with RA and 29·7% in healthy controls (P<0·001). In winter, the prevalence of vitamin D deficiency rose to 80·9% in patients with psoriasis, to 41·3% in those with RA and to 30·3% in healthy controls (P<0·001). Patients with psoriasis or psoriatic arthritis did not differ in 25(OH)D serum levels nor in the prevalence of vitamin D deficiency. In the logistic regression analysis, vitamin D deficiency was associated with psoriasis independently of age, sex, body mass index, calcium, PTH levels and season of blood sampling. A limitation is that the study design does not allow a causal or temporal relationship between vitamin D deficiency and psoriasis to be established. Vitamin D deficiency may be common in patients with psoriasis, especially in winter. © 2011 The Authors. BJD © 2011 British Association of Dermatologists.