In the absence of a related donor, unrelated cord blood transplant (CBT) may be a potential option for patients with a primary immune deficiency (PID). Most published experience consists of single-center data using multiple preparative regimens and GVHD prophylaxis. We report the results of a multicenter prospective trial of unrelated CBT for PID. A total of 24 children with PID, with a median age of 1 year (range: 0.23-7.81 years) and a median weight of 10.5 kg (range: 4-24.4 kg) received unrelated CBT between 1999 and 2003. All patients received a fully ablative conditioning regimen with identical GVHD prophylaxis and supportive care. Most patients (79%) received a 1 or 2 HLA Ag-mismatched cord unit with a median nucleated cell infused of 9.3 x 10(7)/kg (range: 1.0-31.2) and a median CD34 of 2.7 x 10(5)/kg 2.9 (range: 0.6-84.5). The cumulative incidence of neutrophil engraftment by day 42 was 58% (95% CI: 38-79%) at a median of 19 days. Cumulative incidence estimates of grade III-IV acute GVHD at day 100 and chronic GVHD at 1 year were 29% (95% CI: 10-48%) and 24% (95% CI: 3-44%), respectively. The probability of survival at 180 days and 1 year was 66.7% (95% CI: 44.3-81.7%) and 62.5% (95% CI: 40.3-78.4%), respectively. Unrelated CBT should be considered in children with PID.