To understand the underlying genetic and environmental sources of individual variation in basal cortisol levels, we collected salivary cortisol at awakening and at six fixed time points during the day in adult twins and their singleton siblings. Reported time of awakening was verified with heart rate and body movement recordings. Cortisol data were available for 199 MZ twins, 272 DZ twins and 229 singleton siblings from 309 twin families. No differences in cortisol means and variances were found between twins and singleton siblings. Additionally, the correlations for DZ twins and siblings were not significantly different, indicating generalizability of twin study results to the general population. Genetic model fitting showed heritability for cortisol levels during the awakening period (34% for cortisol level at awakening and 32% for cortisol level at 30 min after awakening) but not for cortisol levels later during the day. The current study shows that, while cortisol levels in the awakening period are influenced by genetic factors, cortisol levels throughout most of the day are not heritable, indicating that future gene finding studies for basal cortisol should focus on the first hour post-awakening. © 2005 Elsevier Ltd. All rights reserved.