Actuarial risk assessment instruments for sexual offenders are often used in high-stakes decision making and therefore should be subject to stringent reliability and validity testing. Furthermore, those involved in the risk assessment of sexual offenders should be aware of the factors that may affect the reliability of these instruments. The present study examined the interrater reliability of the Risk Matrix 2000/s between one field rater and one independent rater with a sample of more than 100 sexual offenders. The results indicated good interrater reliability of the tool, although reliability varies from item to item. A number of factors were identified that seem to reduce the reliability of scoring. The present findings are strengthened by examining interrater reliability of the tool in the usual practitioner context and by calculating a range of reliability statistics. Strategies are suggested to increase reliability in the use of actuarial tools in routine practice.