Since 1988 over 10 000 simultaneous cadaveric pancreas-kidney transplants (SPK) have been performed in the United States among patients with end-stage renal disease due to Type 1 diabetes (T1DM). The two aims of this study were to assess the impact on kidney allograft survival of (i) SPK versus transplantation of a kidney alone (KA), and (ii) SPK prior to versus after initiation of chronic dialysis. This retrospective, non-concurrent cohort study examined registry data collected from 8323 patients waitlisted in the United States for an SPK and transplanted with either an SPK or a KA during January 1, 1990 - October 31, 2002. SPK recipients had an adjusted hazard ratio for kidney allograft loss of 0.63 (95% CI: 0.51-0.77, p < 0.001) compared to transplantation without pancreas allograft. SPK recipients who received their allografts prior to beginning chronic dialysis had a lower rate of kidney allograft loss than SPK recipients who received their transplant after initiation of chronic dialysis (adjusted hazard rates (HR) = 0.83, 95% CI: 0.69-0.99, p = 0.042). Simultaneous transplantation of pancreas-kidney compared to kidney transplantation alone and SPK prior to the initiation of chronic dialysis compared to SPK after initiation of dialysis were both associated with longer kidney allograft survival.