From our natural history data, it has been shown that patients with CMV disease had a CMV PCR load ranging from 14,000 to 203 million (median 175,500). The lower bound of the 95% confidence limits of this distribution was 37,000 copies/ml and we aimed to initiate therapy in time to prevent CMV viral load reaching this value. To give a margin of safety, bearing in mind the 1 day average doubling-time of CMV and the timing of sampling twice-weekly, we therefore recommended that preemptive therapy be given once the viral load increases above 3,000 copies/ml. In the past, all patients with a CMV PCR load between 200 and 3,000 copies/ml have received preemptive treatment because the previous PCR assay did not give a quantitative result. As treatment is associated with side effects such as neutropaenia (ganciclovir) and renal impairment (foscarnet) it would be preferable to avoid unnecessary exposure where possible. This study aims to determine: a) whether those patients with 'low level' viral load results (between 200 and 3,000 copies/ml) could be monitored as opposed to starting preemptive therapy with valganciclovir, ganciclovir and/or foscarnet; b) whether those patients with 'high level' viral load results (above 3,000 copies/ml) could stop preemptive therapy earlier, thus maximising the benefits of therapy and minimising its risks.