Korotkoff sounds (KSs) are the clinical norm for noninvasively estimating systemic arterial blood pressure (BP) in children and adults. Their existence and reliability in children <3 years old was unknown. This study addressed the presence and accuracy of KSs in children <3 years. Measurements of the first KS (K1), K4, and K5 (the commonly used sounds) were compared with invasive measurements of systolic and diastolic BP in > or =3 inflations/subject. Subjects were prospectively divided by age <1 (n = 20), 1 to 12 (n = 29), and 13 to 36 months (n = 17). KSs were audible with low frequency in subjects <1 month old and they were excluded from further analysis. In 143 inflations, K1 was audible in 125, K4 in 104, and K5 in 114; the frequency of audible KSs increased with subject age. Reliability of KSs as a surrogate for measured systolic and diastolic BP was assessed using paired t tests. K1 was not significantly different for the entire group or any subgroup. K4 was significantly different in all age groups. K5 was significantly different in only subjects <6 months. In conclusion, K1, K4, and K5 are routinely audible and K1 and K5 provide a reliable estimate of systolic and diastolic BP in children age 1 to 36 months.