Background: Seclusion is a controversial intervention. Efficacy with regard to aggressive behaviour has not been demonstrated, and seclusion is only justified for preventing safety hazards. Previous studies indicate that nursing staff factors may be predictors for seclusion, although methodological issues may have led to equivocal results. Objective: To perform a prospective cohort study to determine whether nursing staff characteristics are associated with seclusion of adult inpatients admitted to a closed psychiatric ward. Method: We studied the association between nurses' demographics and incidence of seclusion during every shift. Data were collected during five months in 2013. Multiple logistic regression was used for analysis. Results: In univariable analysis, we found a non-significant association between seclusion and female gender, odds ratio (OR) = 5.27 (0.98–28.49) and a significant association between seclusion and nurses' large physical stature, OR = 0.21 (0.06–0.72). We found that physical stature is the most substantial factor, although not significant: ORadjusted = 0.27 (0.07–1.04). Conclusion: Nurses' gender may be a predictor for seclusion, but it seems to be mediated by the effect of physical stature. We used a rigorous, census-based, prospective design to collect data on a highly detailed level and found a large effect of physical stature of nurses on seclusion. We found nurses' physical stature to be the most substantial predictor for seclusion. These and other factors need to be explored in further research with larger sample size.