In a recent study we found that testing as a final activity in a skills course increases the learning outcome compared to spending an equal amount of time practicing. Whether this testing effect measured as skills performance can be demonstrated on long-term basis is not known. The research question was: does testing as a final activity in a cardio-pulmonary resuscitation (CPR) skills course increase learning outcome when assessed after half a year, compared to spending an equal amount of time practicing? The study was an assessor-blinded randomised controlled trial. A convenient sample of 7th semester medical students attending a mandatory CPR course was randomised to intervention course or control course. Participants were taught in small groups. The intervention course included 3.5 h skills training plus 30 min of skills testing. The practice-only control course lasted 4 h. Both groups were invited to a retention assessment of CPR skills half a year later. Participants included 89/180 (50%) of those invited to participate in the study. Mean performance score was 75.9 (SD 11.0) in the intervention group (N = 48) and 70.3 (SD 17.1) in the control group, effect size 0.4. The difference between groups was not statistically significant, P = 0.06. This study suggests that testing as a final activity in a CPR skills course might have an effect on long-term learning outcome compared to spending an equal amount of time practicing the skills. Although this difference was not statistically significant, the identified effect size of 0.4 can have important clinical and educational implications.