BACKGROUND: Psoriasis affects a considerable proportion of women in their reproductive years. Limited published data exist about the possible negative impact of the disease itself in the prognosis of pregnancy. On this background, the emergence of newer biologic agents for psoriasis treatment - such as ustekinumab - raises safety issues concerning the exposure to the drug during pregnancy. To our knowledge this is the first report in the literature describing a pregnancy outcome under ustekinumab treatment.OBSERVATION: We report a 35-year-old female psoriasis patient who was under treatment with ustekinumab for a year when she inadvertedly became pregnant. The drug was discontinued immediately and the patient did not opt for termination. During the 12th week of gestation she experienced a spontaneous abortion.CONCLUSION: Although the patient's profile fulfilled 2 general risk factors for spontaneous abortion - she was a smoker and this was her third pregnancy - one could not underestimate the possible role of the drug and of psoriasis per se in this adverse pregnancy outcome. Pregnancy registries and large prospective studies are needed in order to determine whether poorer pregnancies outcomes in psoriatic women are due to the disease itself, associated comorbidities or side-effects of new therapies such as ustekinumab.