To assess the effectiveness of the search for an unrelated donor on the outcome of patients with high-risk acute lymphoblastic leukemia, we analyzed prospectively 136 patients who underwent a search for cord blood (CB) and an unrelated volunteer donor (UD) at the same time. The probability of finding a donor was 58.2%, 70.3%, and 75.7% at 3, 6, and 12 months, respectively. The median time to find a donor was 1.8 months for CB and 3.5 months for UD. Of the 99 patients with a donor, 38.4% failed to undergo the transplant because of a relapse observed at a median of 4 months from the start of the search. In univariate analysis, absence of relapse during the search (p < 0.0001) and transplant (p = 0.004) showed a positive impact on long-term survival. In multivariate analysis, relapse during the search remained the key factor affecting survival (p < 0.0001). Since an extension of the search beyond 3 months enables only a slight increase in the probability of finding a donor compared to the increased risk of relapse, the time of the search should not exceed the 3-month time point. The simultaneous search for CB and UD increases the likelihood of performing a timely transplant.