This study was conducted to assess knowledge, perception, and attitudes regarding cancer and treatment among healthy relatives of cancer patients who attended an outpatient cancer clinic with their relatives who suffer from cancers. The participants recruited in this cross-sectional, interview-based study were 846 (557 female and 289 male subjects) healthy relatives of cancer patients from the outpatient cancer clinic at King Abdulaziz University Hospital, Jeddah, Saudi Arabia. Most of the participants answered that they believed the causes of cancer were genetic (44.90 %), followed by environmental factors (30.10 %), diet (26.90 %), other causes (26.90 %), envy (26.90 %), and black magic (17.60 %). Most of the healthy participants believed that doctors should tell patients the full truth about the diagnosis (83.57 %). More than half of the healthy population stated that cancer patients should accept all types of treatment (chemotherapy and/or radiotherapy and/or surgery), with more male subjects having this position than females (P = 0.014). Most of the participants believed that cancer cannot be caught from another person who suffered from cancer (67.50 %). Most of the participants believed that cancer education was sufficient (66.70 %), with a significant difference between male and female respondents (P = 0.004). With regard to why cancer patients hide their disease, most of the participants in the age group <25 years believed that the causes were fear of loss of health insurance (56.20 %), followed by job loss (34.40 %), and then social stigma (9.40 %); in the age group between 25 and 45 years, the causes were fear of loss of health insurance (76.50 %), followed by social stigma (14.70 %), and then job loss (8.80 %); while in the age group >45 years, the reasons were job loss (47.10 %), followed by health insurance loss (41.20 %), and then social stigma (11.80 %), with a significant difference between groups (P = 0.034). This study demonstrated that still a large number of healthy participants had deficient perceptions and poor attitudes about important issues concerning cancers such as different mode of treatments, alternative treatment, biological causes, and prognosis, particularly among male respondents. Prevention education strategies should be considered, including targeted approaches that aim to reduce disparities in cancer perception among the general population.