The prevalence, stability, and impact of blood contamination in children's saliva on the measurement of three of the most commonly assayed hormones were examined. Participants were 363 children (47% boys; ages 6-13 years) from economically disadvantaged families who donated saliva samples on 2 days in the morning, midday, and late afternoon. Samples (n=2178) were later assayed for cortisol (C), testosterone (T), and dehydroepiandrosterone (DHEA). To index the presence of blood (and its components) in saliva, samples were assayed for transferrin. Transferrin levels averaged 0.37 mg/dl (SD=0.46, range 0.0-5.5, Mode=0), and were: (1) highly associated within individuals across hours and days, (2) positively correlated with age, (3) higher for boys than girls, (4) higher in PM than AM samples, and (5) the highest (>1.0 mg/dl) levels were rarely observed in samples donated from the same individuals. Transferrin levels were associated with salivary DHEA and C, but less so for T. As expected, the relationships were positive, and explained only a small portion of the variance. Less than 1% of the statistical outliers (+2.5 SDs) in salivary hormone distributions had correspondingly high transferrin levels. We conclude that blood contamination in children's saliva samples is rare, and its effects on the measurement of salivary hormones is small. Guidelines and recommendations are provided to steer investigators clear of this potential problem in special circumstances and populations.