A withdrawal syndrome consisting of nervousness, palpitations, diaphoresis, and insomnia appeared in three patients within 16--48 hours following discontinuation of guanabenz, a new centrally acting antihypertensive agent. A similar syndrome of sympathetic overactivity has been described with abrupt withdrawal of clonidine. Three of four patients treated with 48 mg/day but none of 20 other patients treated with 32 mg/day or less guanabenz developed this syndrome. None of the three patients developed hypertensive crisis, though one had a modest rise in the blood pressure above baseline levels. It is concluded that guanabenz therapy should not be discontinued abruptly and that, when possible, the dosage should be limited to less than 48 mg/day.