A isotachophoretic method with conductivity detection was developed and validated to directly determine fenoprofen in human serum. The leading electrolyte contained hydrochloric acid (10 mmol/l), 6-aminocaproic acid (pH 4.8) and polyvinylpyrrolidone (0.1%). The terminating electrolyte was 4-morpholineethanesulfonic acid (5 mmol/l). The calibration curve was linear over the concentration range 0.02-0.40 mmol/l. Within-day standard deviation ranged from 0.001 to 0.004 and between-day standard deviation ranged from 0.001 to 0.004. The limit of determination was 0.02 mmol/l. The assay was employed to determine serum concentration of fenoprofen in patients.