The efficacy and safety of long-term administration of diltiazem hydrochloride were investigated in eight patients with essential hypertension given the drug for approximately five years. Subjective symptoms improved within one to two months after therapy was begun, and no new symptoms appeared. A generally satisfactory blood pressure reducing effect was achieved with no evidence of the escape phenomenon developing over the course of long-term therapy. No exacerbations in ECG findings, cardiothoracic index, or heart rate were observed. Long-term administration of diltiazem had no adverse effects on urinalysis; hematological values; metabolism of protein, lipids, and glucose; renal or hepatic function; or electrolytes. It is thus concluded that diltiazem hydrochloride can be safely given for long periods to patients with hypertension.