ObjectiveTo measure diurnal variation in the CD4 cell count and T-cell reactivity of drug users.DesignA prospective epidemiological study among HIV-infected and non-infected drug users attending the Municipal Health Service of Amsterdam, The Netherlands.PatientsEleven HIV-infected and seven non-infected drug users.Main outcome measuresCD4 cell counts and T-cell reactivity three times a day. T-cell subsets and T-cell reactivity were determined from blinded samples within 2 hours.ResultsThe number of CD4 cells increased by 130 $$ 106/I (P < 0.05) in HIV-infected intravenous drug users over 8 hours. Following stimulation with anti-CD3 monoclonal antibodies, the T-cell reactivity of HIV-infected drug users rose from 118 to 221 c.p.m. (P < 0.01) over 8 hours. CD4 cell counts of the total study population increased by 37% and T-cell reactivity by 93%. The increase in the number of CD4 cells was more marked among active drug users than among drug users who had not used drugs recently.ConclusionVariation in the CD4 cell count and in T-cell reactivity is large among drug users.