Literature trends indicate an increasing awareness regarding the frequency, nature and clinical associations of abnormal and persistent expansions of lymphocytes with cytoplasmic granulation. These particular cells, which represent a minor normal lymphoid subpopulation and are widely referred to as large granular lymphocytes (LGL), generally (but not invariably) express monoclonal antibody-defined membrane NK-associated (NKa) determinants and appear to functionally correspond to those populations involved in cellular cytotoxicity. Increased proportions or absolute numbers of blood lymphocytes with LGL morphology and/or NKa+ phenotypes are associated with a diverse spectrum of clinical (haematological and non-haematological) disorders and may be broadly viewed as secondary (acute and chronic reactive) or primary in nature. Both primary and secondary LGL/NKa+ expansions may be persistent in type and the clinical distinction between the two may be difficult. A number of investigators have proposed schemes for the classification of these disorders but, because of their diversity, abnormal LGL/NKa+ expansions often defy rigid compartmentalisation. This communication examines the general basis of these classifications and illustrates their limitations by reviewing the data for 97 patients recorded in the largest (Yorkshire Leukaemia Group) survey to date of persistent LGL/NKa+ expansions.