Background  Chronic plaque psoriasis is frequently associated with metabolic disorders including obesity. Antitumour necrosis factor α treatments can induce body-weight increase in patients with psoriasis. Information on the effect of ustekinumab on body weight is not available.

Objectives  To investigate whether therapy with ustekinumab is associated with changes in body mass index (BMI) in patients with chronic plaque psoriasis.

Methods  A prospective, multicentre study comparing the changes in BMI in two closed cohorts of patients with psoriasis during 7-month treatment with ustekinumab (n = 79) or infliximab (n = 83).

Results  Patients treated for 7 months with infliximab showed a significant (P < 0·001) increase in mean BMI (2·1 ± 4·5%) and body weight (2·5 ± 3·3 kg) compared with patients treated with ustekinumab (0·1 ± 3·3%; 0·6 ± 1·1 kg). Some 45% of patients treated with infliximab had a BMI increase > 2%, compared with only 11% of those receiving ustekinumab (P = 0·01). In the multivariate analysis, all other clinical parameters predicted the BMI increase, except for the use of infliximab. At month 7, 96% of patients treated with infliximab and 82% of patients treated with ustekinumab achieved at least a 50% improvement from their baseline psoriasis area and severity index (PASI 50), and 69% of the infliximab group compared with 58% of the ustekinumab group achieved at least PASI 75. There was no difference in the proportion of PASI 50 and PASI 75 responders between the two groups.

Conclusions  In contrast to infliximab, ustekinumab does not increase BMI in patients with chronic plaque psoriasis. This difference could be taken into account in the selection of biologics when treating patients with psoriasis.