Mediastinal involvement was found in 11 (group A) of 43 patients affected with acute lymphatic leukemia when early thoracic roentgenograms of these patients were reviewed. Several clinicobiological characteristics of these patients were compared with those with a normal mediastinum as shown in their roentgenograms (group B). Statistically significant differences were observed between group A and group B not only as far as age was concerned (13 years group A, 7.4 years group B) but also in the ratio males/females (10:1 group A, 10:20 group B) and with regard to the leukocyte counts (266 x 10(9)/liter group A, 20 x 10(9)/liter group B). In addition, patients from group A showed a greater 'tumoral mass' and a more prominent extrahematological involvement (45% group A, 15% group B). In these cases 'convoluted' cells were frequently discovered and the blastic cells exhibited significantly lower scores of PAS-positivity and more marked acid phosphatase activity than those in group B. Although the rate of complete remissions (CR) obtained in both groups was similar (80% group A, 93% group B), marked differences were observed not only in the duration of CR but in the period of survival after CR as well, both factors being more prolonged in group B patients. 50% of patients with mediastinal involvement (group A) relapsed in the first 6 months of the evolution of the disease.