Prognostic factors in acute lymphoblastic leukemia (ALL) are used for treatment stratification of ALL. Definition of simple parameters such as the presence or absence of peripheral leukemic cells after one week of treatment could help for stratification. A retrospective study was conducted on 79 previously untreated adult patients with ALL followed in the Hematology department of Hotel Dieu from 1981 to 1991. 84% of patients achieved complete remission (CR), 7% were refractory to induction treatment, and 7 patients (9%) died during the first month after diagnosis. After multivariate analysis the only independent statistically significant factors for achieving CR were the absence of peripheral blast cells at day 7 (PBC D7) (p = 0.009) and age (< 50 years) (p = 0.03). For CR duration the same independent statistically significant factors were found (PBC D7 = 0 versus > 0, p = 0.008; and age < or > or = 30 years, p = 0.045). The PBC D7 value was more significant when circulating blast cells were present at diagnosis. In patients with more than 50,000 PBC at diagnosis, the 10- years event free disease was 62% +/- 20% when PBC were absent at day 7 versus 0% when PBC were present (p < 0.002). All 20 patients with prolonged DFS had PBC D7 = 0 achieving CR by 28 days. The persistence of PBC at Day 7 could be used as a factor to identify a subgroup of poor prognosis adults with ALL.