A simple micellar liquid chromatographic procedure is described to determine tamoxifen in plasma. To perform the analysis, tamoxifen solutions were diluted in water and UV-irradiated for 20 min to form the photocycled derivative with a phenanthrene core which shows intense fluorescence. Samples were then directly injected, thus avoiding long extraction and experimental procedures. The resolution from the matrix was performed with a mobile phase containing 0.15 M SDS-7% n-butanol at pH 3 running at 1.5 mL/min through a C18 column at 40 degrees C. Detection was carried out by fluorescence, and the excitation and emission wavelengths were 260 and 380 nm, respectively. The chromatographic analysis time was less than 15 min. The analytical methodology was validated following the International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) guidelines. The response of the drug in plasma was linear and in the 0.5-15 microg/mL range, with r(2) > 0.999. Accuracy and precision were <9% in both cases. The limits of detection and quantification (in nanograms per millilitre) were 50 and 150 in plasma, respectively. The method developed herein shows no interferences by endogenous compounds. Finally, the analytical method was used to determine the amount of tamoxifen in the plasma of several breast cancer patients from a local hospital.