Salivary cortisol may be used as a biomarker of stress and anxiety in children with an autism spectrum disorder (ASD). Some suggestions have been made that the measurement of cortisol needs to be undertaken by repeated days' observations to ensure reliability of the data obtained. These requirements are discussed in regard to 14 studies of the test-retest agreement and stability in cortisol data across repeated daily measurements. Results of those studies almost universally fail to support the argument for repeated daily measurements of cortisol. Implications for the research protocols of studies using cortisol as an index of stress in children with ASD are discussed.