The recent growth of pen-based devices, such as the Personal Digital Assistant (PDA), offer mobility and a more natural interface than that of a conventional computer. The feasibility and application of the PDA for mood and cognitive assessment were investigated by examining possible interactions of individual characteristics and administration medium. Previous studies have provided evidence that individual characteristics of 'computer anxiety' and 'private self-consciousness' divergently covaried with mood scores measured by computer and paper methods. To investigate the relationship between individual characteristics and medium effects, 136 paid participants were allocated to and completed mood assessment tasks and a short battery of cognitive tasks by either the computer, PDA or the paper method. Self-ratings of mood measured by these three modalities covaried divergently with measures of computer anxiety and private self-consciousness. In addition, computer anxiety covaried with reaction time on the visual search task obtained on computers, but there was no such relationship when measured by a PDA. These results show that computer anxiety can affect the results of assessments of cognitive function as well as of mood ratings, and suggest that pen-based systems may have advantages over conventional computers in this respect.