We retrospectively reviewed multiparameter flow cytometric analyses in 50 peripheral T-cell neoplasms (PTCNs). Results were interpreted within the context of a large cohort of nonneoplastic T-cell populations. All PTCN diagnoses were confirmed with morphologic and/or molecular analysis. Aberrant populations were defined as discrete immunophenotypic clusters exhibiting loss of or increased or diminished expression of T-cell antigens relative to internal immunophenotypically normal T-cell populations. An antigenic pattern was considered abnormal if it exceeded ranges for T-cell subsets in specific anatomic sites or was not normally encountered. Forty-six of 50 and 41 of 50 demonstrated 1 or more and 2 or more aberrations, respectively. The most common abnormally expressed antigen was CD3, followed by CD7, CD5, and CD2. Except for CD7, abnormally dim or bright antigen expression was more common than deletion. Only 3 cases were abnormal solely based on expansion of an otherwise immunophenotypically normal population; the remainder had patterns of antigen expression not seen in nonneoplastic populations. These data indicate that most PTCNs are aberrant by multiparameter flow analysis. However, results must be interpreted within the context of thorough knowledge of the immunophenotypic spectrum of nonneoplastic T cells.