To investigate antibody maturation and serum levels of cytomegalovirus (CMV) DNA after primary CMV infection, we studied 51 immunocompetent and 27 kidney-transplant patients. Compared with the immunocompetent patients, the transplant patients had significantly more-prolonged and -variable antibody maturation, clearly longer durations of viremia, and higher levels of CMV DNA; however, antibody maturation continued for >1 year even in immunocompetent patients. Long-term ganciclovir prophylaxis in the transplant patients was associated with either delayed immunoglobulin-G seroconversion, inhibition of antibody maturation (n=2), or immunoglobulin-class switching (n=1). In conclusion, antibody maturation continues in immunocompetent patients for a period longer than previously had been thought and is significantly delayed or even inhibited in kidney-transplant patients.