Objective Inflammatory bowel disease (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), shares clinical and immunological features with psoriasis. Genome-wide association studies have found common susceptibility genes. However, epidemiologic data evaluating the association between psoriasis, psoriatic arthritis and risk of IBD are sparse. We aimed to evaluate the association between psoriasis, psoriatic arthritis and incident CD and UC among women in the USA.

Methods 174 476 women were enrolled in the Nurses’ Health Study (NHS) (1996–2008) and NHS II (1991–2007). Lifetime history of physician-diagnosed psoriasis and psoriatic arthritis was confirmed by supplementary questionnaires. Information on CD and UC was obtained by self-reported questionnaires and confirmed by medical record review.

Results We documented 188 incident cases of CD and 240 incident cases of UC during follow-up. Psoriasis was associated with a significantly increased risk of subsequent CD with a multivariate-adjusted relative risk (RR) of 4.00 (95% CI 1.72 to 9.27) for NHS and 3.76 (1.82 to 7.74) for NHS II. By contrast, we did not observe a significant increase in risk of UC associated with psoriasis. In a pooled analysis of both cohorts, women with psoriasis experienced a significantly increased risk of CD (RR, 3.86, 95% CI 2.23 to 6.67), but not UC (RR, 1.17, 95% CI 0.41 to 3.36). The risk of CD was especially pronounced among psoriatics with concomitant psoriatic arthritis (RR, 6.43, 95% CI 2.04 to 20.32).

Conclusions Psoriasis with concomitant psoriatic arthritis is associated with an increased risk of incident CD.