An inception cohort of 87 patients with lupus nephritis was evaluated using a classification tree regression technique. Four relevant outcomes were studied: (1) renal insufficiency (serum creatinine > 5.0 mg/dl); (2) renal failure; (3) death due to renal involvement; and (4) any death due to systemic lupus erythematosus. All 4 outcomes could be predicted by one or more renal severity measures (serum creatinine, 24-h urinary protein excretion, nephrotic syndrome, or duration of prior renal disease), and among those with nonsevere renal disease, with a single disease activity measure (the National Institutes of Health or le Riche index). In general 3 prognostic groups (high, intermediate and low risk) could be identified for each outcome. Our results demonstrate the value of regression tree techniques in studies of prognosis and are compatible with a hypothesis of the interaction of disease activity with organ damage in lupus nephritis.