The detection of preformed donor-specific alloantibodies (DSA) with multiplex-bead arrays has led to the common observation that individuals without a history of pregnancy, transfusion or transplantation can have circulating anti-HLA antibodies of unknown etiology. We retrospectively analyzed the risk of antibody-mediated rejection (AMR) and graft outcome in 41 kidney transplant recipients with DSA of unknown etiology (DSA cause-unk) at the time of transplantation. Twenty-one patients received a posttransplantation desensitization protocol, and 20 received standard immunosuppressive therapy. The mean number of DSA was 1.4 ± 0.8, ranging from 1 to 5. Complement-dependent cytotoxicity crossmatches were negative for all the patients. Flow cytometry crossmatches were positive in 47.6% of cases. The incidence of acute AMR was 14.6% at 1 year, regardless of the immunosuppressive regimen. No patients experienced graft loss following AMR. At month 12, across the entire population of patients with DSA cause-unk, the outcomes were favorable: the measured glomerular filtration rate was 63.8 ± 16.4 mL/min/1.73 m2, the screening biopsies showed low frequencies of microvascular inflammation and no transplant glomerulopathy, and graft and patient survival were 100%. In conclusion, patients with DSA cause-unk are able to mount AMR but have favorable 1-year outcomes.