Nitisinone is a reversible inhibitor of 4-hydroxyphenylpyruvate dioxygenase and an active substance in the orphan drug used for the treatment of hereditary tyrosinemia type I, a rare genetic disease caused by mutations in the gene for the enzyme fumarylacetoacetase. The aim of our study was to develop a simple and accurate RP–HPLC method with UV detection for routine determination of nitisinone in commercially available pharmaceutical forms. The chromatographic separation was achieved on a reversed-phase column Purospher STAR® RP–8 end–capped (150 x 4.6 mm i.d., particle size 5 μm), with a mobile phase consisted of a mixture of acetonitrile and water acidified with o-phosphoric acid with pH adjusted to 3.0, 65:35 (V/V), filtered through 0.45μm nylon filter. The flow rate was kept at 1 mL/min. A diode array detector measured the UV absorbance at 272 nm. The injection volume was 10 μL. The method was validated by a determination of system suitability, specificity, linearity, range, accuracy, precision, detection limit and quantitation limit. Then, the method was applied for determination of nitisinon in commercially available capsules. The proposed RP-HPLC method allows a simple, accurate, precise and rapid determination of nitisinone in pharmaceuticals. The advantages of the method include simple sample treatment, good precision (RSD less than 2%) and high recovery (greater than 99%). The method could be recommended for routine analysis in quality control laboratories, in stability studies as well as for the evaluation of potentially counterfeit capsules containing nitisinone. Key words: Nitisinone; High performance liquid chromatography; Method validation; Hereditary tyrosinemia type I