Extreme thrombocytopenia, purpura, and melena developed in a 71-year-old woman four days after ethambutol administration was started. The macrophage inhibition factor test was positive for ethambutol, and the thrombocytopenia subsided shortly after discontinuation of the drug therapy. The sequence of events in this patient suggests that, although previously unreported, thrombocytopenia should be added to the list of side effects of ethambutol.