Studies assessing hypothalamic-pituitary-adrenal axis functioning in young children commonly involve parental collection of salivary cortisol in ambulatory settings. However, no data are available on the compliance of parents in collecting ambulatory measures of children's salivary cortisol. This study examined the effects of parental compliance on the cortisol awakening response (CAR) and diurnal cortisol slopes in a sample of preschoolers. Eighty-one parents were instructed to collect their child's salivary cortisol samples upon their child's waking, 30 and 45 minutes post-waking and before bedtime on two weekdays. Subjective parental compliance was assessed using parent-report, and objective parental compliance was assessed using an electronic monitoring device. Rates of compliance were higher based on parent-report than electronic monitoring. Parental noncompliance as indicated by electronic monitoring was associated with higher waking cortisol and lower CAR. Findings suggest the need to incorporate electronic monitoring of parental compliance into developmental neuroendocrine research, especially when assessing the CAR.