Septicaemias are frequent and severe in patients with acute leukaemia under aplastic treatment. The present study concerns 69 such patients: 29 with acute lymphoblastic leukaemia (ALL), and 40 with acute non-lymphoblastic leukaemia (ANLL). All were treated in single rooms in the same hospital and in similar conditions. The overall incidence of septicaemia was 62%; it was 60% in patients with recently diagnosed ALL and 68% during relapses. More than 34% of ALL patients and 82.5% of ANLL patients had one or several episodes of septicaemia. Among the 74 pathogens isolated 50% were Gram-positive organisms, 45% Gram-negative organisms and 5% Candida spp.. The first episodes of septicaemias were predominantly caused by Gram-positive spp. (61%) and the subsequent ones by Gram-negative spp. (60%). The primary infection could only be diagnosed in 19% of the cases and was most frequently located in the digestive tract or perineal region. The most common focal complications were lung infections (18 cases), skin infections (12 cases) and septic shock (15 cases). Seventy-four p. cent of the patients survived with prompt and potent antibiotic therapy. Death occurred in 26% and was clearly related to the following factors: chemotherapy of relapsed leukaemia and/or blastic aplasia and/or successive episodes of septicaemia. The incidence and severity of septicaemias in leukaemic patients will only be reduced by improved prophylactic measures against infection and by less pronounced and shorter chemotherapy-induced granulocytopenia.