This is an open-label, multicenter, Phase I/II study of the safety of escalating doses of single-agent PRO131921 in patients with chronic lymphocytic leukemia (CLL) who have relapsed after or were refractory to treatment with a purine nucleoside analogue-containing regimen. The trial will enroll in two phases: a Phase I dose-escalation portion and a Phase II expanded treatment cohort, with enrollment of additional patients in order to expand safety experience and collect preliminary anti-leukemia activity data.