The direct determination of lansoprazole by using a flow injection analysis (FIA) with UV-detection and its application to the pharmaceutical capsules is described, in this study. The best carrier solvent was found to be 0.01 M NaOH and it was determined at optimum conditions such as flow rate of 1 ml min(-1) and wavelength of 292 nm. Examining the repeatability of the method that was found to be 1.72% for intra-day and 2.13% for inter-day precision using the 8.01 x 10(-6) M lansoprazole concentration has validated the method. The linear range of the method was 5.4 x 10(-6) to 5.4 x 10(-5) M. The limit of detection and quantification was found to be 5.8 x 10(-7) and 1.7 x 10(-6) M, respectively. The proposed method was applied to the pharmaceutical capsules and very good results obtained. Thus, the FIA method for the quantification of lansoprazole can be proposed as a cheap, rapid, easy, accurate, and precise method for the routine determination in pharmaceutical preparations. (C) 2004 Elsevier B.V. All rights reserved.