We present a unique case of infectious mononucleosis attended with transient asymptomatic BK virus (BKV) manifestation in the urine of an immunocompetent caucasian boy without kidney dysfunction. The urine sediment showed abundant decoy cells initially misdiagnosed as malignant cancer cells. This case demonstrates that the occurrence of polyoma-BKV bearing decoy cells is self-limiting and not necessarily associated with overt kidney disease in an immunocompetent child. The shedding of decoy cells into the urine might be promoted by viral co-infections modulating the host's immune response such as infectious mononucleosis.