Background. Acute rejection (AR) has been shown to be a significant risk factor for chronic rejection (CR) in kidney transplant recipients, yet many recipients with AR do not progress to CR. The purpose of this study was to determine if certain AR episodes are associated with a worse prognosis. Methods. The study group consisted of 279 kidney transplant recipients, all treated for a single episode of biopsy-proven AR. All AR episodes were initially treated with steroids; steroid-resistant rejection was managed with an antibody preparation. Results. First, by univariate techniques, we determined the clinical impact of severity of AR (as estimated by delta creatinine [dCr], defined as the change in baseline serum creatinine level 6 weeks after AR treatment) on two different endpoints - biopsy-proven CR and graft survival. Irrespective of 6-week dCr, all recipients with AR had a significantly increased risk of CR vs. those with no AR (P<0.01). Recipients with dCr between 0.5 and 1.0 mg/dl had a significantly higher incidence of CR vs. those with dCr <0.5 mg/dl (P<0.05), but a significantly lower incidence vs. those with dCr >1.0 mg/dl (P<0.05). We then performed multivariate analysis. We used severity of AR in addition to other variables (e.g., timing of AR, donor age) to determine which factors were most associated with risk for CR and graft loss. Risk for CR increased with AR episodes occurring >6 months after transplant (relative risk [RR]=3.8, P=0.005); with moderate or severe (vs. mild) AR episodes (RR=2.7, P=0.05); and with dCr >0.5 mg/dl (vs. <0.5 mg/dl) at 6 weeks after AR treatment (RR=2.3, P=0.1). Findings were similar when graft survival (death-censored) was the endpoint instead of CR. Conclusions. All AR episodes are associated with some increase in the risk for CR. But AR episodes occurring >6 months after transplant and those of increased severity (as assessed qualitatively by histologic grading and quantitatively by dCr) confer the greatest risk. Recipients with these risk factors could be targeted with measures to decrease their risk for CR, including trials of novel immunosuppressive regimens.