Fifty-six renal allograft recipients were studied prospectively for 3 months or longer after transplant. The polymerase chain reaction (PCR) was used to screen peripheral blood leucocyte (PBL) specimens for CMV, human herpesvirus 6 (HHV6) and human herpesvirus 7 (HHV7) DNA (DNAemia) in 67 healthy controls and i n serial (fortnightly) PBL specimens from the 56 allograft recipients. None of the healthy controls had detectable CMV DNAemia, although HHVG and HHV7 DNAemia was found in 7% and 9% of individuals respectively. In contrast, DNAemia due to CMV, HHVG and HHV7 was found i n 50%, 36% and 39% of patients respectively, at some time during the post-transplant period. Of the 28 patients who had CMV DNAemia, eight developed "CMV disease." The risk of progression t o "CMV disease" was increased in patients with concurrent DNAemia to all three viruses (relative risk 3.7; 95% CI 1.3-10.5). The relative risk of "CMV disease" for patients with concurrent CMV and HHV7 was also increased (RR = 3.5; 95% CI = 1.1-1 1.6), while the association between CMV and HHVG was inconclusive (RR = 2.1; 95% CI = 0.7-6.6). The first 26 patients recruited to the study also had serial serum samples tested for antibody responses to the three viruses. "CMV disease" was associated with rising antibody titres to HHV7 (Fisher's exact test, P = 0.021, and weakly so with HHVG ( P = 0.07). It is concluded that in patients with CMV DNAemia, concurrent infection/reactivation of HHV7 (and possibly HHV6) is associated with an increased risk of progression t o "CMV disease."