This paper analyses the factors associated with successful radiofrequency ablation (RFA) of lung metastases. The study group comprised 37 patients [19 female, mean age 61 (34-83)] with 72 metastases who had follow-up CT scans available for analysis and for those with no recurrence >6 months follow-up. Internally cooled electrodes were used in 64 and expandable electrodes in 8. The tumour size and location, electrode type, number of ablations, duration of ablation, year of treatment and tumour contact with vessels larger than 3 mm were recorded. The mean tumour diameter was 1.8 cm (0.4-6.6 cm). Mean follow-up in those without recurrence was 13.1 months (6-48). Recurrence was common in larger tumours, occurring in 7/7 (100%) tumours >3.5 cm compared with 18/65 (28%) < or = 3.5 cm (P < 0.01). Recurrence occurred in 14/24 (58%) tumours in direct contact with large vessels compared with 11/48 (23%) of the remainder (P = 0.04). On multivariate analysis, size was the dominant feature (P = 0.013); vessel contact and peripheral location did not reach significance (P = 0.056 and 0.054 respectively). Peripheral tumours less than 3.5 cm with no large vessel contact are the optimal tumours for RFA.