Flow cytometric analysis of peripheral blood (PB) S + G2/M phase size was performed in 73 adult patients with untreated acute non-lymphoblastic leukaemia, to assess whether the results may correlate to response rate and patient prognosis. All patients were treated with the same induction chemotherapy regimen: ARA-C alone or in combination with an anthracycline antibiotic. Pretreatment PB S + G2/M phase size is significantly correlated to induction response rate (p less than 0.02), duration of response (p less than 0.02) and duration of survival. Patients with low PB S + G2/M phase size experience a longer survival, in patients over and below 50 yr (p less than 0.001). Lastly, early deaths tend to be more frequent in the high median age and high PB S + G2/M phase size group. Our study suggests that PB S + G2/M phase size has prognostic significance in obtaining response and duration of survival.