Introduction. Psoriasis is a relatively common chronic inflammatory disease. It clinically manifests as raised, well defined erythematous plaques with irregular borders and silvery scales. Psoriasis appears to be mediated by abnormal immune system functioning, including T lymphocyte and macrophage activation and release of various cytokins, such as interleukin 12 (IL-12) and IL-23. Recently a new biologic agent Ustekinumab has been used in the treatment of psoriasis. Our aim in this study was to assess the efficacy and tolerability of ustekinumab in moderate to severe psoriasis vulgaris and to observe and report any adverse reaction. Material and Methods. Thirty five psoriatic patients above the age of 18 years having moderate to severe psoriasis were included in this study. Ustekinumab is available in pre-filled syringe 45mg/0.5ml, 90mg/1.0ml for subcutaneous injection according to body weight at the intervals of 0, 4 weeks, and then every 12 weeks. It is given in hospital by a doctor or specialist nurse. The assessment of the patients’ condition and improvement was carried out after administering each dose using PASI score. Results. Thirty five patients were included in this study. Baseline PASI score of our patients ranged from 11.4 to 39.8 (mean: 21.1).There was a dramatic response to treatment with ustekinumab in which PASI decreased to 6.7 after the second dose, followed by subsequent responses that reached 2.6 after the 6th dose. After the second dose, 61% of the cases had marked improvement and 11% had clearance of their skin lesions. After the last, sixth dose there was a marked improvement in 65% of cases and the percentage of complete clearance increased to 24%. Ustekinumab had positive effect on psoriatic nail changes as well-there was a significant improvement in 50% of cases and complete clearance (cure) in 24% of cases. Conclusion: Ustekinumab is effective in the treatment of severe and resistant cases of psoriasis vulgaris. It is well tolerated by the patients. No reactions or serious side effects have been reported.