The European Organization for Research and Treatment of Cancer Core Quality of Life Questionnaire (EORTC QLQ-C30) is a well-validated instrument that assesses health-related quality of life (HRQOL) in cancer patients. It is used in cancer clinical trials in Europe, Canada, and the United States, and has demonstrated high reliability and validity in different groups of cancer patients. Despite thorough testing of reliability and validity, we have not identified any reports on its test/retest reliability; thus, a test/retest study was performed at the Norwegian Radium Hospital (NRH). Cancer patients from the outpatient clinic who were off treatment for > or = 3 months were eligible for the study. The EORTC QLQ-C30 was given to the patients when they presented for their visit. The second questionnaire was received by the patients 4 days later. Of 291 eligible patients, 270 (93%) agreed to participate and 190 (73%) completed both questionnaires. The test/retest reliability measured by Pearson's correlation coefficient was high for all functional scales, with a range from .82 for cognitive and role function to .91 for physical function. The r value for global HRQOL was .85. For the symptom scales--nausea/vomiting, fatigue, and pain--the coefficients were .63, .83, and .86, respectively. The single-item coefficients ranged from .72 for diarrhea to .84 for financial impact. The Spearman rank correlation was in the same range for all dimensions. The EORTC QLQ-C30 seems to yield high test/retest reliability in patients with various cancer diagnoses whose condition is not expected to change during the time of measurement.