This study sought to evaluate serology and PCR as tools for measuring BK virus (BKV) replication. Levels of immunoglobulin G (IgG), IgM, and IgA against BKV capsids were measured at five time points for 535 serial samples from 107 patients by using a virus-like particle-based enzyme-linked immunosorbent assay. Viral DNA in urine and plasma samples was quantitated. The seroconversion rate was 87.5% (14/16); 78.6% (11/14) and 14.3% (2/14) of patients who seroconverted developed viruria and viremia, respectively. Transient seroreversion was observed in 18.7% of patients at 17.4 ± 11.9 weeks posttransplant and was not attributable to loss of antigenic stimulation, changes in immunosuppression, or antiviral treatment. Titers for anti-BK IgG, IgA, and IgM were higher in patients with BKV replication than in those without BKV replication. A rise in the optical density (OD) of anti-BK IgA (0.19), IgM (0.04), or IgG (0.38) had a sensitivity of 76.6 to 88.0% and a specificity of 71.7 to 76.1% for detection of viruria. An anti-BK IgG- and IgA-positive phenotype at week 1 was less frequent in patients who subsequently developed viremia (14.3%) than in those who subsequently developed viruria (42.2%) (P = 0.04). Anti-BK IgG OD at week 1 showed a weak negative correlation with peak urine viral load (r = -0.25; P = 0.05). In summary, serial measurements of anti-BKV immunoglobulin class (i) detect onset of viral replication, (ii) document episodes of seroreversion, and (iii) can potentially provide prognostic information.