Naturalistic research methods have been developed to collect data in the daily environment, providing ecological valid measures. Recent reports suggest, however, that compliance with fixed time sampling protocols may be problematic and can bias results. This study investigated compliance with an intensive, random time sampling protocol for salivary cortisol and effects of non-compliance on cortisol results. Twenty female twin pairs and nineteen of their sisters were instructed to take saliva samples when signaled at ten unpredictable moments on each of five consecutive days. Subjects recorded collection times, unaware that compliance with the sampling protocol was being investigated by means of electronic monitoring devices. Samples taken <= 15 min after the signal, according to self-report, were defined as adherent to the protocol. Samples taken <= 10 min after the self-reported collection time, according to the monitor, were defined as accurate.