The current study quasi-experimentally assessed the outcome of high-intensity inpatient sex offender treatment in the Netherlands in terms of sexual and violent (including sexual) recidivism. It was hypothesized that treated sex offenders would show lower recidivism rates than untreated sex offenders of the same risk level. In line with the risk principle of the Risk, Need, Responsivity (RNR) model, we predicted that this would especially hold true for offenders of higher risk levels. The study sample consisted of 25% of all convicted Dutch sex offenders not referred to any form of treatment and discharged from prison between 1996 and 2002, and all convicted Dutch sex offenders referred to inpatient treatment who were discharged between 1996 and 2002. Static-99R risk levels of these 266 offenders were retrospectively assessed and survival curves regarding sexual and violent (including sexual) recidivism were compared between treated and untreated offenders, controlling for level of risk. Mean follow-up was 148.0 months (SD = 29.6) and the base rate of sexual recidivism was 15.0% and 38.4% for violent (including sexual) recidivism. Cox regression survival analyses showed marginally significant lower failure rates regarding sexual recidivism for treated high-risk offenders only, and significantly lower failure rates regarding violent (including sexual recidivism) for treated sex offenders of moderate-high and high-risk levels. No treatment effects for low and low-moderate risk offenders were found. Results underscore the risk principle of the RNR model: Treatment is more effective when its dosage is attuned to risk level.