PURPOSE We evaluated the effect of Internet-based cognitive behavioral therapy (iCBT), with or without therapist support, on the perceived impact of hot flushes and night sweats (HF/NS) and overall levels of menopausal symptoms (primary outcomes), sleep quality, HF/NS frequency, sexual functioning, psychological distress, and health-related quality of life in breast cancer survivors with treatment-induced menopausal symptoms. PATIENTS AND METHODS We randomly assigned 254 breast cancer survivors to a therapist-guided or a self-managed iCBT group or to a waiting list control group. The 6-week iCBT program included psycho-education, behavior monitoring, and cognitive restructuring. Questionnaires were administered at baseline and at 10 weeks and 24 weeks postrandomization. We used mixed-effects models to compare the intervention groups with the control group over time. Significance was set at P < .01. An effect size (ES) of .20 was considered small, .50 moderate and clinically significant, and .80 large. RESULTS Compared with the control group, the guided and self-managed iCBT groups reported a significant decrease in the perceived impact of HF/NS (ES, .63 and .56, respectively; both P < .001) and improvement in sleep quality (ES, .57 and .41; both P < .001). The guided group also reported significant improvement in overall levels of menopausal symptoms (ES, .33; P = .003), and NS frequency (ES, .64; P < .001). At longer-term follow-up (24 weeks), the effects remained significant, with a smaller ES for the guided group on perceived impact of HF/NS and sleep quality and for the self-managed group on overall levels of menopausal symptoms. Additional longer-term effects for both intervention groups were found for hot flush frequency. CONCLUSION iCBT, with or without therapist support, has clinically significant, salutary effects on the perceived impact and frequency of HF/NS, overall levels of menopausal symptoms, and sleep quality.