Purpose Three classification and prognostic systems exist for primary cutaneous B-cell lymphoma (PCBCL). Whereas the WHO classification defines its categories based on histology, both the European Organisation for Research and Treatment of Cancer classification and the conventional staging system consider skin site. We sought to incorporate both histology and skin site into a single prognostic index designed to predict survival in patients with PCBCL. Methods The associations between histology, skin site, and survival were determined using adjusted proportional hazards analysis conducted on 926 patients with PCBCL identified in the Surveillance, Epidemiology, and End Results data, spanning 1973 to 2001. Results Four primary CBCL prognostic index (CBCL-PI) groups were identified. Group IA included indolent histologies involving any skin site. Group IB included diffuse large B-cell histology involving favorable skin sites (head/neck, arm). Group II included diffuse large B-cell histology involving unfavorable skin sites (trunk, legs, disseminated) or immunoblastic large B-cell histology involving favorable skin sites. Group III included immunoblastic large B-cell histology involving unfavorable skin sites. The adjusted mortality hazards ratios were 1.0, 1.3 (95% CI, 0.99 to 1.7), 2.1 (95% CI, 1.6 to 2.7), and 4.5 (95% CI, 2.8 to 7.2) for groups IA/B to III, respectively. The corresponding 5-year relative survivals were 94%, 86%, 60%, and 34%. Conclusion The CBCL-PI identifies adverse combinations of histology and skin site, helping to reconcile differences in current classification and prognostic systems for PCBCL.