Outcome results of observational databases are frequently criticized as relying on incomplete information from incomplete patient populations. Few data are available to dispute these arguments of selection bias. The European Group for Blood and Marrow transplantation (EBMT) decided to address this question by evaluating the hematopoietic stem cell transplants performed in 1993. A comprehensive survey was launched in an effort to collect informations on all transplants for hematological malignancies performed throughout Europe during the year 1993. The main goals of this effort were to compare the group of spontaneously reported patients with the group of retrospectively solicited patients, and to give an accurate estimate of the outcome of all patients. For the year 1993, the annual EBMT activity survey indicated 6336 transplants performed for hematological malignancies in Europe. A total of 5651 transplants could be analyzed; 2595 were reported spontaneously by the teams (group A) and 3056 were retrieved on solicitation (group B). Patients and transplant characteristics for group A and B were very similar for most parameters with a few exceptions. There was no statistical difference for outcome at 3 years between groups A and B: disease-free survival (DFS) was 45 +/- 1% and 44 +/- 1%, relapse incidence (RI) 41 +/- 1% and 42 +/- 1%, transplant-related mortality (TRM) 23 +/- 1% and 23 +/- 1%, and overall survival (OS) 54 +/- 1% and 55 +/- 1%, respectively, for group A and group B. The real outcome at 3 years for the 5651 patients (group A + group B) transplanted in 1993 was 44 +/- 1%, 41 +/- 1%, 23 +/- 1%, and 54 +/- 1%, for DFS, RI, TRM and OS, respectively. The outcome at 3 years by transplant modality, autologous or allogeneic transplants, and by disease categories showed no difference between groups A and B.