Biomarkers of clinical response to rituximab (RTX) therapy and early predictors of outcome are still under investigation. We report a flow cytometric immunophenotyping analysis from peripheral blood leukocyte subpopulations of two patients with systemic lupus erythematosus (SLE) associated thrombocytopenia and one patient with rheumatoid arthritis (RA), before and after 6 weeks of treatment with RTX. Our results show a reduced population of CD19(+) expressing cells (B cells) after RTX treatment in all three patients. Increased frequency of peripheral regulatory CD4(+)CD25(high) T cell subset and the CD3(-)CD16(-)CD56(bright) NK cell subset after RTX therapy were also observed in all patients, the latter being more pronounced in the SLE patient with sustained clinical response. In addition, an increased population of NKT cell subsets was observed in the patients with clinical response. This is the first evaluation of NK and NKT cells as biomarkers of clinical response after rituximab therapy in rheumatic diseases