The effects of a mobile telephone conversation on driving were studied in the advanced driving simulator at VTI. Twenty subjects, 10 men and 10 women, between 60 and 71 years and 20 subjects, also 10 men and 10 women, between 23 and 58 years participated in the study. The road the subjects drove could be characterized as 'easy'. It was straight and not expected to cause the subjects any problems with speed choice and steering strategy. The workload imposed on the subjects by the driving task was thus supposed to be very low. The telephone task included handling of the telephone and a conversation, containing a working memory part and a decision part. The handling task consisted of pushing the handsfree button to activate the telephone when it was calling. During the conversation the subjects were asked to listen to pre-recorded sentences and for each sentence to judge if they experienced it as 'sensible' or 'nonsense'. After a number of sentences they were required to recall the last word in each sentence, in the order they were presented. Effects are discussed based on hypotheses like: When drivers' are solving the telephone task: their ability to control the vehicle, for example keep a consistent lateral position, will deteriorate; their workload will increase, due to the addition of the telephone task, and lead to a reduction in speed; the hypothetical effects are predicted to be stronger for elderly people. Implications for traffic safety and for future in-car information systems are also discussed.