Inconsistent and incomplete outcome reporting may make estimates of treatment effects from published randomized trials unreliable. We aimed to determine outcome reporting practices and source of differences in reporting quality among randomized trials of primary immunosuppression in kidney transplantation. We searched the Cochrane Renal Group's Specialized Register, 2000–2012, specified four core outcomes we expected trials to report, and recorded if and how completely each was reported. We identified 179 trials. One hundred sixty-eight (94%) reported death, 145 (81%) as number dead and 119 (66%) as time to death. One hundred sixty-five (92%) reported graft loss, 158 (88%) as number with graft loss and 127 (71%) as time to graft loss. One hundred twenty-one (68%) reported creatinine and 114 (64%) estimated GFR (eGFR). One hundred forty-one (79%) provided complete reports of number dead, 95 (53%) censored and 99 (55%) uncensored number with graft loss. Seventy-three (41%) provided complete reports of time to death, 67 (37%) censored and 31 (17%) uncensored time to graft loss. Complete reporting of graft function was infrequent: 62 (35%) eGFR and 50 (28%) creatinine. All four outcomes were reported in any form in 61 (34%) and completely in 28 (16%) trials. No single trial or journal characteristic was consistently associated with complete outcome reporting. Outcome reporting in kidney transplant trials is inconsistent and frequently incomplete, and published estimates of treatment effects may be unreliable.