Abstract Ustekinumab, a monoclonal antibody against interleukin (IL)-12 and IL-23, is used for the treatment of psoriasis and psoriatic arthritis. The IL-12 pathway also plays an important role in regulating immunity to Mycobacterium tuberculosis. A multicentre, double-blind, randomized clinical trial study of 121 Korean patients with moderate-to-severe psoriasis revealed at least 75% improvement in Psoriasis Area and Severity Index (PASI) score after 12 weeks of ustekinumab treatment. Ten cases of latent tuberculosis infection (LTBI) were reported. Lupea-Chilom et al. [Lupea-Chilom DS, Solovan CS, Farcas SS et al. Latent tuberculosis in psoriasis patients on biologic therapies: real-world data from a care center in Romania. Medicina (Kaunas) 2023; 59: 1015] conducted a retrospective observational study on a total of 97 patients with severe psoriasis who were on different biologics. That study showed that two of seven patients on monotherapy ustekinumab developed LTBI. In this abstract, we present the case of a 49-year-old White woman with resistant moderate-to-severe chronic psoriasis without psoriatic arthritis who developed LTBI while on ustekinumab. She was diagnosed with psoriasis in 2012 with involvement of the scalp, flexural, vulval and gluteal regions as well as the nails. She was started on ustekinumab in October 2013 following a satisfactory prebiologics screen (including a negative QuantiFERON test). Her PASI score was zero at week 12. In 2017, she developed a malar rash and positive antinuclear antibody test, which was not tested prior to starting biologics. As lupus is a potential adverse effect of this medication, the ustekinumab was stopped in April 2017. Further rheumatological investigations ruled out lupus and ustekinumab was restarted in November 2017. As part of the screening process prior to restarting treatment, she had a repeat QuantiFERON test, which was positive. The respiratory team reviewed the patient and issued a 3-month course of rifampicin and isoniazid to treat LTBI. Following a few months of monitoring, it was deemed safe to restart ustekinumab in December 2018 and her PASI improved. In February 2020, due to the COVID pandemic, this medication had to be stopped. She was started on acitretin as an alternative in 2021 with a resulting worsening of her condition. In December 2021, the patient was restarted on ustekinumab. In this abstract, we illustrate that ustekinumab has the potential to reactivate latent tuberculosis, a phenomenon that is infrequently reported and attributed to its impact on the IL-12 pathway. To address this concern, we recommend conducting repeated annual screening during treatment in addition to a comprehensive baseline evaluation, including a chest X-ray and QuantiFERON test. Additionally, it is advisable to consider initiating isoniazid chemoprophylaxis concurrently with ustekinumab treatment as a precautionary measure.