Studies on sexual behavior frequently require that subject volunteers engage in intrusive/sensitive assessment procedures. While earlier investigators have found that these demands may result in volunteer bias (volunteers differing from nonvolunteers), these studies were limited to nonclinical samples. The present study involved 182 males admitted to an inpatient alcoholism rehabilitation program. Those patients who volunteered to participate in an intrusive study examining sexual functioning were dissimilar to nonvolunteers. Compared to nonvolunteers, volunteers reported a greater interest in sex, less satisfaction with sex, more concerns about sexual functioning, a greater incidence of premature ejaculation, and a greater incidence of negative sexual experience. Volunteers also had a higher MAST score, used outpatient substance-abuse counseling more often, and more frequently had a diagnosis of cocaine/amphetamine dependence and cannabis dependence. These differences between volunteers and nonvolunteers suggest a need to use caution when generalizing the results of similar studies to the overall population. Procedures that may help to minimize volunteer bias are offered.