BACKGROUND: While hands-on training is a prerequisite for successful education of nursing students, constraints on clinical training availability and quality have increased focus on effects of in-school simulation training. However, existing research has produced inconsistent conclusions and the literature lacks high-powered evidence from controlled trials. OBJECTIVES: To test effects of a simulation scheme on student professional self-confidence in technical and non-technical skills, as well as to investigate effects on knowledge acquisition and interaction with clinical training. DESIGN: Field experiment, treatment is a three + two day simulation training scheme while control is a standard three hour simulation session. Self-confidence in a list of technical and non-technical skills is measured in three survey-rounds. Enriched with data on type of clinical training site and grade attainment. SETTING: University College Copenhagen Department of Nursing, all third year students in 2019. PARTICIPANTS: 352 in cohort, out of which 316 participated and 311 answered first survey round (163 in treatment, 148 in control). METHODS: Field experiment analyzed utilizing multivariate OLS regression analysis. RESULTS: Students who receive increased simulation training report markedly higher levels of professional self-confidence immediately after training. This effect is double the size for confidence in technical skills, compared to non-technical skills. The effects on self-confidence in technical skills persist at the end of the following semester for those that receive low intensity clinical training. Students who receive the treatment see a small (and statistically uncertain) relative increase in grade attainment in the semester of treatment, but this difference dissipates over time. CONCLUSIONS: Simulation training has substantial positive short-term effects for the professional self-confidence of nursing students and appears to have small positive effects on knowledge acquisition. Most of these effects are crowded out by other factors (notably intensive clinical training) over time but might have long-term positive effects for those that do not receive other intensive hands-on experiences. This is interpreted as an indication that simulation training can be used to compensate for uncertainties in providing sufficient training experiences outside of academic training.