OBJECTIVE To analyse the expression of myeloid markers and CD34 antigen on lymphoblasts in adult ALL and its relationship with prognosis. METHODS Immunophenotypes were examined using indirect immunofluorescence method in 102 de novo ALL. RESULTS The incidence of myeloid antigen expression in adult ALL was 21.6% and the commonest one was CD33 (15.7%). There was a higher incidence of myeloid antigens expression in ALL-L2 than in ALL-L1 (25.6% vs 5.3%, P = 0.05). CD34 was expressed in lymphoblasts from 30 of 56 patients (53.6%). Incidence of CD34 expression in B-ALL was higher than that in T-ALL (61.7% vs 11.1%, P < 0.01). No relationship between CD34, myeloid antigens and cell maturity was found within B-ALL. There was no relation between expression of myeloid antigens and CD34. The CR rate in My(+)-ALL was lower than that in My(-)-ALL (52.6% vs 80.0%, P < 0.025), and was no relation with CD34 expression. In addition, Ph chromosome and/or bcr/abl fusion gene was positive in 35.9% of the patients, and CR rate of Ph positive patients was higher than that in Ph negative patients. CONCLUSIONS Expression of myeloid antigens was related to FAB subtype and cell maturity in adult ALL. There was no relationship between myeloid antigen expression and CR rate. A higher incidence of CD34 expression was found in Pro-B-ALL than in common-ALL and Pre-B-ALL. Expression of CD34 had no relation with CR rate.