Colour discrimination was assessed by means of the Farnsworth-Munsell 100-Hue test in 54 patients being treated for tuberculosis with ethambutol and 50 patients who were treated for tuberculosis by drugs other than ethambutol. The ethambutol group showed significantly more errors than controls along the deuteran axis. The sub-group of ethambutol treated patients who had been taking the drug for more than 2 months showed significantly more errors along the tritan axis. It is concluded that subclinical colour discrimination impairment is relatively common in patients receiving ethambutol and that a deuteran type of defect is an early and a tritan type of defect is a later manifestation of ethambutol toxicity.