The most frequently discussed ethical issue on the subject of cancer is whether patients should be told their diagnosis. Cancer patients' knowledge of their diagnosis and the relationship between patient characteristics and that knowledge were evaluated in a cross-sectional study. From July 1992 to December 1993 at Tri-Service General Hospital, Taipei, a total of 964 patients with cytologically- or pathologically-proven cancer were studied. Of those studied, 359 patients (37.2%) knew their diagnosis. The proportion of patients who believed that they had a benign tumor was 25.1%; 12.6% believed they had nonneoplastic disease (inflammation or other chronic disease); 7.5% believed they had precancerous changes; and 17.6% knew nothing about their diagnosis. Multivariate analyses showed that patients with the following characteristics were inclined to know their diagnosis: younger age (20-59 yr), higher levels of education (> 9 yr) with relatives of a high socioeconomic status, cancer of a genital organ, cancer requiring disfiguring treatment (osteosarcoma, leukemia), cancer in obvious sites (head and neck), cancer of more than one type (multiple primary lesions), cancer of good prognosis (excluding lung, liver, gall bladder or pancreatic cancers), aggressive anticancer treatment and longer duration of pathologic proof (> 14 d). The results showed that concealing the diagnosis from cancer patients is still very common in Taiwan. Patient's sociodemographic background and disease-related factors were the most important patient characteristics related to their knowledge of the diagnosis.