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.