The treatment outcome of a high-intensity inpatient sex offender treatment program was evaluated by comparing the sexual recidivism rates of 472 treated and 282 untreated sex offenders. The program is designed for moderate- to high-risk sex offenders and follows the principles of effective correctional treatment. The current investigation is an extension of an earlier study (Nicholaichuk et al., 2000) with the addition of 176 participants, an extra 4 years follow-up, and the use of Cox regression survival analysis to control for three potentially confounding variables: age of release, sexual offending history, and length of follow-up. Treated offenders sexually recidivated significantly less than the comparison group over nearly 20 years of follow-up, even after controlling for the aforementioned variables. The substantive findings suggest that treatment adhering to the "what works" principles can reduce long-term sexual recidivism for a moderate- to high-risk group of sex offenders. (Contains 2 figures and 5 notes.)