A quantitative modification of the shell vial assay was used to investigate cytomegalovirus viremia in solid-organ transplant recipients. The level of viremia detected in 109 of 407 specimens ranged from 0.02 to 28 infectious foci per 100,000 leukocytes. By using a Poisson model, a technique was developed to determine 95% confidence limits for the measured levels of viremia. These confidence limits were used to determine the level of viremia that could be excluded by culturing a given number of cells. Longitudinal assessment of two transplant recipients revealed different patterns of viremia and demonstrated that significant disease sometimes occurred with low-level viremia. On the basis of the results of the studies, culture of at least 4 x 106 leukocytes is recommended for the sensitive detection of cytomegalovirus viremia.