Morphological, cytochemical, immunological and clinical features of 19 adult patients with T cell acute lymphoblastic leukemia (T-ALL) were investigated at the time of diagnosis and were compared with those of 34 adult patients affected by B cell acute lymphoblastic leukemia (B-ALL). Immunophenotypic studies employing a wide panel of monoclonal antibodies (mAbs) revealed a heterogeneous pattern of antigen expression in the five T-ALL groups that were identified on the basis of blast T cell differentiation levels. PAS (periodic acid Schiff) negativity and focal AP (acid phosphatase) positivity, as well as white blood cell count and serum lactic dehydrogenase levels, were significantly related to T-ALL when compared with B-ALL. On the contrary, no statistically significant difference was demonstrated in the clinical outcome.