We describe herein two cases of sick sinus syndrome possibly due to lopinavir-ritonavir in HIV-infected individuals. The heart rate dropped to 30 to 40 beats per minute in both cases, but patients remained asymptomatic and recovered promptly after discontinuation of lopinavir-ritonavir. The time until onset varied; one patient developed bradyarrhythmia 9 days after the initial dose, and another 4 hours after. Since lopinavir-ritonavir is a frequently used antiretroviral agent, clinicians must be aware of this potentially lethal adverse effect.