The objective of the study was to investigate the influence of the blood concentrations of hydroxychloroquine ([HCQ]) and its derivative desethylhydroxychloroquine ([DHCQ]) on lymphocyte activation or differentiation in HCQ-treated lupus patients. We studied the correlations between [HCQ], [DHCQ], and the frequency of various lymphocyte subsets in 58 HCQ-treated lupus patients (mean HCQ dose: 4.93 +/- 1.58 mg/kg/day; mean duration of the disease: 122 +/- 64 months). [HCQ] and [DHCQ] were determined by high-performance liquid chromatography (HPLC). Lymphocyte markers were studied by flow cytometry using monoclonal anti-CD3, -CD4, -CD8, -CD25, -DR, -CD45RA, -CD45RO, -CD19, -CD38, and -CD86 antibodies. sIL2-R serum concentrations were measured by enzyme-linked immunosorbent assay (ELISA). [HCQ] and [DHCQ] were 599.9 ng/mL (median: 529.5; range: 55-1935) and 353.43 (median: 286 ng/mL; range: 118-1090). In a multiple regression analysis, [HCQ] and [DHCQ] were associated with the HCQ prescribed dose in mg/kg/day (P = 0.0002 and P = 0.03) and with compliance to the treatment (P = 0.004 and P = 0.03). We found a negative correlation between [HCQ], [DHCQ], and the CD45RO+ cell frequency among CD3+CD4+ cells (P = 0.03 and P = 0.007, respectively). Other lymphocyte subset markers (LSMs) and sIL2-R concentrations were not significantly associated with [HCQ] or [DHCQ]. In the multiple regression analysis, CD45RO+ expression was negatively influenced by [HCQ] (P = 0.005), and positively influenced by smoking habits (P = 0.005) and age (P = 0.005). Similar results were found in the multivariate model including [DHCQ]. Disease activity and taking more than 10 mg/day of corticosteroids or an immunosuppressive drug did not influence CD45RO+ expression. Lupus patients had less CD3+CD4+CD45RO+ cells than controls (P = 0.03). In lupus patients, HCQ and DHCQ may alter the generation or the blood circulation of CD4+CD45RO+ lymphocytes in a concentration-dependent pattern.