Background: Risk of radiation exposure from standard computed tomography (CT) surveillance is a major concern in the management of stage I seminoma testis patients. The TRISST randomized trial (NCT00589537) demonstrated that effective monitoring could be achieved with a reduced scan schedule or using Magnetic Resonance Imaging (MRI) instead of CT. Here, TRISST data is used to evaluate the economic consequences and health outcomes of different surveillance schedules in seminoma patients in the UK. Methods: 669 men were randomized to 4 surveillance groups undergoing 7 CTs over 5 years, 3 CTs over 3 years, 7 MRIs, or 3 MRIs of the retroperitoneum. Resource use (including investigations, scans, hospitalisations and treatment for relapse) and health outcomes data (EQ-5D 3L) on each patient were collected at baseline and over a period of 6 years after randomization. Health resources were costed using publicly available national unit costs. Within-trial mean total costs and quality-adjusted life years (QALYs) were estimated for each alternative schedule. Under the perspective of the UK health system, cost-effectiveness was evaluated using a cost per QALY gained framework. Probabilistic sensitivity analysis was used to reflect parameter uncertainty and evaluate results robustness. Results: Since only 82 men (12%) relapsed, most health resource consumption (76%) happened during the disease-free period. Patients undergoing 7 MRIs yielded, on average, slightly higher health benefits (5.20 QALYs) but at higher costs (GBP £6,632, see table). Compared to 7 CTs, 7 MRIs was estimated to have 63% probability of being cost-effective at a system willingness to pay threshold of £20k/QALY. 3 MRIs had similar costs and benefit to 7 CTs, whereas 3CTs was more expensive than 7 CTs and 3 MRIs, providing marginal additional benefits. Conclusions: Overall, small differences exist in total costs and total QALYs between different strategies. A 7 scan MRI schedule yielded more health benefits than other strategies, but at higher costs. Considering possible capacity constraints with MRI, the reduced radiation exposure relative to CT, and non-inferiority for clinical outcomes in the primary analysis, a 3 scan MRI schedule may be the best option to replace current CT-based longer surveillance practice. Clinical trial information: NCT00589537.