BACKGROUND: Economic evaluations that include the patient perspective often base their estimates of patient time and travel costs on data collected at a single point in time. This, however, may be inaccurate if the costs of accessing care change substantially over time, as may be the case for young people in transition from paediatric to adult health services. AIMS: The aim of this study was to explore the differences in these time and travel costs between two data collection points for young individuals in transition between health care services, and thus to provide an insight of whether such costs should be collected more than once. METHODS: Descriptive statistics and regression modelling were used to estimate the average difference in costs between the two points of data collection, as well as the potential drivers of those cost differences. RESULTS: We found a small difference in costs between the two time points, equal to -£45.78 [95% CI: - 89.70 to - 1.86]. The results were largely driven by changes in the unit cost of visits and in the number of attendances. CONCLUSIONS: A simple and common assumption that patient costs could be collected at a single time point cannot be made in the context of our study. When deciding on the frequency of elicitation of patient costs, future studies should consider the relative impacts of additional data collection on the estimates of efficiency, inequalities and resource implications for collecting new data.