Objectives: Low dose computerised tomography (CT) screening for lung cancer can reduce lung-cancer-specific mortality. The objective of this study was to analyse healthcare costs and healthcare utilisation of participants in the Danish lung cancer CT-screening trial (DLCST). Materials and methods: This registry study was nested in a randomised controlled trial (DLCST). 4104 participants, current or former heavy smokers, aged 50-70 years were randomised to five annual low dose CT scans or usual care during 2004-2010. Total healthcare costs and healthcare utilisation data for both the primary and the secondary healthcare sector were retrieved from public registries from randomisation - September 2011 and compared between (1) the CT-screening group and the control group and, (2) the control group and each of the true-positive, false-positive and true-negative groups. Results: The median annual costs per participant were significantly higher in the CT-screening group (Euros [EUR] 1342, interquartile range [IQR] 750-2980) compared with the control group (EUR 1190, IQR 590-2692) (p< 0.0001). When the cost of the CT-screening programme was excluded, there was no longer a statistically significant difference between the CT-screening group (EUR 1155, IQR 567-2798) and the control group (p= 0.52). Analyses according to the diagnostic groups showed that annual costs were 10.57 (95% CI 7.09-15.75) times higher for the true-positive and 1.67 (95% CI 1.20-2.32) times higher for the false-positive group compared with the control group. Conclusion: Low dose lung cancer CT screening increases healthcare costs compared with no screening; this difference was attributable to the costs of the CT-screening programme. Overall healthcare costs were higher for the true-positive and false-positive groups than for the control group, also when excluding the cost of the CT-screening programme. This increase was outweighed by the larger true-negative group showing no significant differences in costs compared with the control group.