The objective of this study was to evaluate the significance of indeterminate HIV test results in the prison setting. No specific information or guidelines are currently available to direct counseling of incarcerated persons with an indeterminate HIV test. A medical chart review was conducted on all incarcerated inmates at the Rhode Island State Prison who received indeterminate HIV test results between the inception of mandatory testing in 1990 and October 1996. Thirty-five inmates had an indeterminate HIV Western blot (WB) result, and 31 had follow-up HIV testing. Twenty-three of 31 (74%) of the prisoners with follow-up HIV tests seroconverted (95% confidence interval, 55%-88%). Drug/alcohol use, including crack cocaine and injection drug use, was strongly associated with seroconversion (p < 0.01, odds ratio [OR] = 11.8, relative risk [RR] = 2.04). Injection drug use was also significantly associated with seroconversion (p = 0.03, OR = 9.3, RR = 1.56). This is the highest rate of seroconversion ever reported for persons with indeterminate WB test results. Indeterminate test results need to be interpreted differently in the prison setting than in the community. Prison inmates with indeterminate HIV serology should be counseled that in all likelihood they are HIV-infected, and confirmatory viral load testing should be conducted immediately.