Whether current suicide risk or a history of attempted suicide is related to the antidepressant effect of a low-dose ketamine infusion remains unclear. In total, 47 patients with treatment-resistant depression (TRD), including 32 with low current suicide risk and 15 with moderate or high current suicide risk, were randomized to groups receiving a low-dose ketamine infusion of either 0.2 or 0.5 mg/kg. Among the patients, 21 had a lifetime history of attempted suicide. Suicide risk was assessed based on the Suicidal scale of the Mini-International Neuropsychiatric Interview. The 17-item Hamilton Depression Rating Scale (HDRS) was used to measure depressive symptoms at baseline, at 40 and 240 min after infusion, and sequentially on Days 2-7 and 14 after ketamine infusion. Generalized estimating equation models indicated that the time effects of both 0.5 and 0.2 mg/kg ketamine infusions were significant during the study period. The models also indicated that current suicide risk (p = .037) but not lifetime history of attempted suicide (p = .184) was related to the trajectory of total HDRS scores. Patients with moderate-to-high current suicide risk benefited more from the low-dose ketamine infusion compared with those with the low current suicide risk. Patients with TRD having moderate or high current suicide risk may be prioritized to receive a low-dose ketamine infusion, which may aid suicide prevention. (PsycInfo Database Record (c) 2024 APA, all rights reserved).