Thirty-three participants in a human immunodeficiency virus (HIV) medication adherence feedback (MAF) intervention were compared with 58 HIV-positive non-participants in laboratory-tested CD4 and viral load. The intervention provided adherence feedback and counselling based on a visual display from an electronic pill bottle (MEMSTM). Multiple regression controlling for baseline CD4 and showed that postintervention CD4 was higher for MAF participants than for non-MAF participants. Non-MAF participants' CD4 significantly declined over time. MAF participants were also less likely than non-MAF participants to have a detectable postintervention viral load.