AbstractWe recently developed enzyme‐linked immunosorbent assays (ELISAs) for the detection of anti‐BK virus IgG‐ and IgM‐antibodies, and also a convenient and rapid serum neutralization test. Complemented by a traditional haemagglutination inhibition test (HAI) these methods were used to investigate the longitudinal response to BKV infection in sequentially taken sera from 29 children under treatment for cancer.In separate experiments it was shown that the results were not at any extent influenced by antibodies against other polyomaviruses.At the time of diagnosis the prevalence of specific IgG‐ and IgM‐antibodies and the geometric mean IgG levels were not significantly different for the patients compared with a group of healthy children. The primary infections seemed to occur at the same age for the two groups of children.Seven of the patients had a primary infection with BKV. The results indicate that the host response in moderately immunosuppressed children during primary infection is the same as expected for healthy individuals with the development of specific IgG, HAI, and NT antibodies, and, usually, production of BKV‐IgM for several months. The results indicated that whether specific IgM was demonstrated in the first sample or appeared later during a reactivation episode, this parameter was correlated with profound immunosuppression.Significant titre changes, detectable IgM antibodies and/or seromnversions were demonstrated in 69% (20/29) of the cancer patients. Such indications of recent viral activity was recorded in 42% (S/i9) of chifdren with meningococcal infections. The observation periods for the two groups of patients are, however, not direct