We have conducted flow cytometric studies of two subsets of lymphocyte markers in groups of migraineurs during (n = 12; group B) and outside (n = 10; group C) of a migraine without aura attack (total n = 22; group A), including a group of patients tested in both of these phases (n = 5; group D), and compared these results with those obtained from a population of age- comparable, sex- and race-matched healthy volunteers (n = 12; group E). Comparison of the first set of lymphocytes (CD3+CD16+56+, CD3-CD16+56+, CD3-CD19+, CD3+CD19+, and CD3+HLA-DR+) between the patients in group A and the controls (group E) showed differences, reflecting greater group A percentages of CD3+CD16+CD56+ and CD3-CD19+ lymphocytes. Furthermore, these differences reached statistical significance only for the CD3+CD16+CD56+ lymphocytes, and then solely for the patients in group C (Scheffe's test, p<0.05). Paired analysis of the above lymphocyte markers for subjects in group D failed to show significant difference