BACKGROUND: Allergic rhinitis (AR) reduces quality of life as a result of impaired psychological well-being and perceived impaired cognitive functioning. Few studies have measured cognitive functions objectively and it remains uncertain whether AR leads to an objective reduction in cognitive functions. OBJECTIVE: The present study investigated whether AR is associated with a decrement in several aspects of cognitive functioning. Furthermore, the study investigated whether AR patients invest more 'mental effort' in order to achieve the same cognitive performances as healthy controls. METHODS: Twenty five patients with seasonal allergic rhinitis (SAR) and 26 healthy controls, matched for age, education and sex, were tested on a battery of time-demanding and strenuous objective cognitive tests and subjective questionnaires, both before and after nasal provocation (NP). The cognitive functions assessed were sustained attention, short- and long-term memory and speed of information processing. Mental effort was assessed using visual rating scales. RESULTS: Sustained but not short cognitive performance was impaired in patients after NP. Patients showed an increased effort on short cognitive tests. CONCLUSION: SAR patients suffer from cognitive performance decrements that can be compensated by additional mental effort for short tasks only.