Aims: It has been shown that the free cortisol level in saliva may reflect plasma free cortisol. The measurement of cortisol in saliva is a simple method, and as such it is important in the pediatric age group. In this research, the diagnostic value of measurement of salivary cortisol (SC) measurement was examined in adrenal insufficiency (AI). Methods: Fifty-one patients, mean age 10.8 ± 4.29, who were investigated for possible AI, were included. Basal cortisol levels were below 18 µg/dl. Adrenal function was determined by low-dose ACTH test. During the test, samples for SC were obtained simultaneously with serum samples (at 0–10–20–30–40 min). Results: Mean basal serum cortisol level was 8.21 ± 4.10 µg/dl (mean ± SD). Basal SC was correlated to basal serum cortisol (r = 0.64, p < 0.001). A cut-off of 0.94 µg/dl for SC differentiated adrenal insufficient subjects from normals with a sensitivity and specificity of 80 and 77%, respectively. A peak SC less than 0.62 µg/dl defined AI with a specificity of 100%; however, sensitivity was 44%. Conclusion: Measurement of SC may be used in the evaluation of AI. It is well-correlated to serum cortisol. Peak SC in low-dose ACTH test can be used to differentiate patients with AI in the initial evaluation of individuals with suspected AI.