In contrast to the accepted general assumption that polyethylene glycol (PEG) is non-immunogenic and non-antigenic, animal studies clearly showed that uricase, ovalbumin and some other PEGylated agents can elicit antibody formation against PEG (anti-PEG). In humans, anti-PEG may limit therapeutic efficacy and/or reduce tolerance of PEG-asparaginase (PEG-ASNase) in patients with acute lymphoblastic leukemia and of pegloticase in patients with chronic gout, but did not impair hyposensitization of allergic patients with mPEG-modified ragweed extract or honeybee venom or the response to PEG-IFN in patients with hepatitis C. Of major importance is the recent finding of a 22 - 25% occurrence of anti-PEG in healthy blood donors, compared with a very low 0.2% occurrence two decades earlier. This increase may be due to an improvement of the limit of detection of antibodies during the years and to greater exposure to PEG and PEG-containing compounds in cosmetics, pharmaceuticals and processed food products. These results raise obvious concerns regarding the efficacy of PEG-conjugated drugs for a subset of patients. To address these concerns, the immunogenicity and antigenicity of approved PEGylated compounds should be carefully examined in humans. With all these data in hand, patients should be pre-screened and monitored for anti-PEG prior to and throughout a course of treatment with a PEGylated compound. Finally, protein conjugates with the poorly immunogenic hydroxy-PEG sequence or other hydrophilic polymers are in early phases of development and may represent an alternative to immunogenic PEGylated proteins.