A 32-year-old farmer had signs and symptoms of dinitrophenol poisoning after crop spraying with a herbicide containing derivatives of 2,4-dinitrophenol. Dinitrophenol causes toxicity by the uncoupling of oxidative phosphorylation in the mitochondria of cells throughout the body. In man the classic syndrome consists of lassitude, malaise, headache, increased perspiration, thirst, and dyspnea which may progress to hyperpyrexia, profound weight loss, respiratory failure, and death. Because dinitrophenol compounds are widely used, it is likely that some patients with unexplained fever have unrecognized dinitrophenol poisoning. Prompt recognition of the clinical manifestations of toxicity is vital for appropriate therapy. We describe a method for detection of dinitrophenol in plasma and urine.