This study evaluates the sensitivity and positive predictive value of some of the morphological flags supplied by the Technicon H-1 haematological analyser to detect circulating blasts. A total of 1,269 venous blood samples from 91 paediatric patients diagnosed of acute leukaemia were analysed in the Technicon H-1. A conventional microscopic manual method was used for reference. The prevalence was 17.7% for the presence of blasts. The results showed that all samples with more than 5% blasts were detected by some WBC morphological flag: only 5 samples (2.2%) with less than 5% blasts showed no morphological flag. In most cases (86%), the flags observed were either the ‘Atyp’ flag or the ‘Blast’ flag or both simultaneously. The positive predictive value obtained by the ‘Blast’ flag for the true presence of blasts was 66.8%, and for the ‘Atyp’ flag 34.9%; it was 75.8% when both flags were present simultaneously. False positives were associated with postchemotherapy monocytosis or severe neutropenia.