Purpose: The aim of this study was to investigate the diagnostic delay and its determinants among oral cancer patients in Tehran, Iran. Materials and Methods: This study was conducted between September 2004 and September 2006 in three university hospitals, and included 100 consecutive patients with primary oral squamous cell carcinoma (international classification of disease, ICD-10 sites C01 to C06). Data were obtained through questionnaire interviews and medical records of the patients were reviewed to obtain information on the date of diagnosis, primary tumour site and the stage of the tumour at the time of diagnosis. Statistical analysis was performed by t test, ANOVA and logistic regression. Results: The mean diagnostic delay was 7.2 months (SD 7.5, range 1 to 36 and median 4). The most important determinants of longer diagnostic delay were being single (OR = 4.8; 95% CI = 1.5 to 14.8; P 0.05) and being at advanced tumour stages (OR = 5.3; 95% CI = 1.8 to 15.6; P 0.01). The mean patient and professional delays were 5.3 months (SD 6.1 and median 2) and 2.1 months (SD 2.1 and median 1), respectively. Patients at advanced tumour stages were more likely to have longer patient and professional delays than those at early stages (OR = 5.6; 95% CI = 1.8 to 17.3 and OR = 3.4; 95% CI = 1.2 to 9.4, respectively; P 0.05). Living alone was also a determinant of longer patient and professional delays (OR = 7.1; 95% CI = 2.0 to 24.7, OR = 3.5; 95% CI = 1.2 to 10.3, respectively; P 0.05). Conclusions: Developing preventive programmes that focus on the enhancement of public and professional awareness about oral cancer is essential to promote earlier diagnosis in Iran. Keywords: diagnostic delay, early diagnosis, Iran, Middle East, oral cancer