Human immunodeficiency virus type 1 (HIV-1) infected patients treated with combination antiretroviral therapy frequently have the level of HIV-1 RNA detectable in plasma driven below the lower limit of detection of current assays, 50 copies ml(-1). Patients may continue to exhibit viral loads (VLs) below the assay limit for years, yet on some occasions the VL may be above the limit of detection. Whether these 'blips' in VL are simply assay errors or are indicative of intermittent episodes of increased viral replication is of great clinical concern. By analyzing the occurrence of viral blips in 123 treated HIV-infected patients, we show that patients do not share a common probability distribution of blip amplitude and thus reject the hypothesis that blips are solely due to assay variation.