Objectives (1) Validate an existing clinical tool for assessing risk after deliberate self-harm (DSH), Manchester Self-Harm Rule, in a new setting and new population, (2) develop a clinical decision rule based on factors associated with repeated self-harm in a Swedish population and (3) compare these rules.

Design A consecutive series of 1524 patients attending one of Scandinavia's largest emergency departments (ED) due to DSH during a 3-year period were included. Explanatory factors were collected from hospital charts and national databases. A nationwide register-based follow-up of new DSH episode or death by suicide within 6 months was used. We used logistic regression, area under the curve and classification trees to identify factors associated with repetition. To evaluate the ability of different decision rules to identify patients who will repeat DSH, we calculated the sensitivity and specificity.

Main outcome measure Repeated DSH or suicide within 6 months.

Results The cumulative incidence for patients repeating within 6 months was 20.3% (95% CI 18.0% to 22.0%). Application of Manchester Self-Harm Rule to our material yielded a sensitivity of 89% and a specificity of 21%. The clinical decision rule based on four factors associated with repetition in the Swedish population yielded a sensitivity of 90% and a specificity of 18%.

Conclusions Application of either rules, with high sensitivity, may facilitate assessment in the ED and help focus right resources on patients at a higher risk. Irrespective of the choice of decision rule, it is difficult to separate those who will repeat from those who will not due to low specificity.