OBJECTIVE: In a recent economic evaluation conducted alongside an RCT, we encountered asymmetry in the time horizons for effectiveness and cost outcomes. The effectiveness variable was evaluated at the end of the trial, but some hospitalizations that started during the RCT continued beyond the end of the trial period. This paper demonstrates how estimates of cost-effectiveness ratios can be very sensitive to this problem at the analysis stage. METHODS: Medical care resource-use data were collected prospectively in an RCT comparing two treatments for a chronic condition characterized by acute episodes often requiring hospitalization. A vector of country-specific unit costs was used to convert resource consumption into monetary values for the purpose of performing a cost-effectiveness analysis. Effectiveness was measured as the number of successfully treated patients (STPs) at the end of the six-month trial. Cost-effectiveness acceptability curves (CEACs) for analyses that include and exclude components of resource consumption beyond the trial are compared. RESULTS: The incremental cost per STP is £10,008 if the components of resource use beyond trial are excluded and -£27,200 if included. This result is due to the effect which exclusion or inclusion of cost data beyond trial has on estimates of incremental costs: £2,640 and -£7,130 respectively. The impact on the CEAC is shown to be profound e.g. for a critical ICER of £500, the probability that the treatment is cost-effective is increased by 0.732 if beyond-trial costs are included. CONCLUSIONS: Producers and consumers of cost-effectiveness evidence need to be aware of the potential problem of asymmetry observed in our study since these results may have significant consequences on decision-making. Economic theory would suggest that the beyond-trial components should be excluded from our base case analysis since they will have had no bearing on the observed number of STPs.