Temporomandibular disorders (TMD) are a significant public health problem, affecting approximately 5–12% of the population. Objectives: This retrospective cross‐sectional study investigated the relationship between 8 AM serum cortisol levels (8ASC) and disc displacement disorders (DDD) of TMD. One hundred and forty patients with DDD were recruited. Among them, 60 patients comprised the case group of disc displacement without reduction with limited opening (DDWORWLO, age 37·7 ± 17·22), and 80 were ‘other DDD’ for the control group (age 36·4 ± 13·08). The independent variables included domains of demography, history, malocclusion, comorbid symptoms, comorbid TMD and 8ASC. Data were analysed with the chi‐square test, logistic regression and receiver operating characteristic (ROC) curve. Results of multiple logistic regression showed that 8ASC was the only factor significantly related to DDWORWLO (P = 0·006). Receiver operating characteristic analysis of DDWORWLO and 8ASC indicated an area under the curve of 0·669, standard error of 0·049 and P value of 0·001. The adequate cut‐off point of 8ASC was 12·45 (μg dL−1), with sensitivity of 0·636, and specificity of 0·729. 8 AM serum cortisol level can be used as a clinical clue to differentiate DDWORWLO from other DDD.