The effect of acute exposure to low-level radio frequency electromagnetic fields (REF) generated by mobile phones on short-term memory and attention was assessed in two experiments. Most of the tests manipulated task difficulty or what might be termed cognitive load. This manipulation is important since previous studies have argued that exposure to mobile phones might affect cognitive functions only under conditions which tax the cognitive system. All participants were exposed to REF (half were exposed to GSM—Global System for Mobile Communication—signals and the other half were exposed to unmodulated signals) in one testing session, while in a separate session participants were exposed to sham signals. To investigate potential lateralised effects, the mobile phone was positioned on the left side of the head for half of the participants and on the right side for the other half. No significant effect of exposure to REF was detected in any of the six tasks used in either the low or high cognitive load conditions. This study used much larger sample sizes than is typical for this type of research and REF exposure was administered under double-blind conditions. Overall, the results indicate that acute exposure to REFs emitted by mobile phones do not have a strong impact on cognitive functions. Copyright © 2007 John Wiley & Sons, Ltd.