More than 1200 patients who received pindolol for the treatment of hypertension, angina pectoris, and various arrhythmias in studies conducted in the United States were included in the New Drug Application submitted to the FDA. Nearly 1000 of these patients received pindolol as monotherapy. The side effects reported were generally transient and of mild or moderate severity. The most frequently reported side effects seen after pindolol administration, compared to those seen after placebo, were in decreasing order of incidence: headache, dizziness, insomnia, muscle pain, fatigue, weakness, nervousness, joint pain, edema, nausea, and muscle cramps. Other side effects that occurred more frequently with pindolol than with placebo but at a rather low incidence induced weight gain, bizarre dreams, visual disturbances, lethargy, and diarrhea. Nasal congestion, throat discomfort, nocturia, impotence, pruritus, anxiety, hypotension, bradycardia, and heart failure occurred only rarely. Of the 323 patients who received pindolol alone for the treatment of mild to moderate hypertension, only 20 (6.2%) were withdrawn from the study because of side effects. Overall, 3.4% of the patients treated with pindolol were withdrawn because of side effects, most of which involved the central nervous system, that is, insomnia, anxiety, dizziness, and headache. However, a few patients manifested some edema and weight gain while receiving pindolol alone. Review of the side effects data did not reveal a tendency for the incidence of side effects to be dose related. One placebo-controlled, double-blind study designed to evaluate the fixed dosages of 15, 30, and 60 mg in the treatment of mild to moderate hypertension suggested that only the incidences of insomnia and nervousness increased with increasing doses. However, these side effects were generally transient and of mild or moderate severity. The evidence indicates that pindolol has an acceptable safety profile and that any side effects that appear are generally well tolerated and disappear with continued treatment.