Background. The design of new diagnostic tests would benefit from knowing patients' preferences for the collection of bodily fluids. These preferences, however, are unknown. The authors assessed patients' preferences for the collection of saliva, urine and blood as diagnostic fluids. Methods. The authors handed out surveys adult outpatients who sought treatment at two medical care sites and one dental care site at a medical center regarding the comfort, convenience and ease of collection of saliva, urine and blood for diagnostic testing. Results. A total of 413 surveys were completed. Subjects regarded the donation of saliva as more comfortable and convenient than that of blood or urine at the doctor's (physician's or dentist's) office, and they reported that saliva and urine are easiest to collect at home compared with blood. Male subjects and subjects who had ever donated saliva were more likely to perceive saliva donation in the doctor's office favorably. Subjects at all care sites regarded the donation of saliva as more comfortable and convenient than that of blood or urine; however, subjects at the dental care site perceived saliva donation more favorably than did subjects at the medical care sites. In addition, respondents said they were more willing to participate in research and medical testing if it required saliva donation rather than urine or blood collection. Conclusions. In terms of convenience and comfort, saliva compared well with blood and urine as a diagnostic fluid for clinical and research testing. Patients who sought care at both medical and dental sites shared these perceptions. Clinical Implications. Given strong patient preferences, saliva may be the fluid of choice for the development and implementation of patient-centered diagnostic tests in research and practice.