Dogs have the ability to determine the direction of an odour trail left by a human. This study examined how much olfactory information from this trail is required by dogs to determine direction. Six dogs, able to determine direction, were tested on a 21 footstep trail laid on 21 individual carpet squares, one footstep per square, by the same individual wearing the same shoes. Dogs brought in at right-angles to the trail at its centre were able to correctly determine direction better than chance (P < 0.025). Dogs were unable to determine direction when the order of the footsteps was randomized by rearranging the order of the carpet squares. When the individual odour cue was removed, but ground disturbance left, dogs were unable to determine direction, indicating that it was the odour of the individual that was used to determine direction. In the final experiment the number of footsteps made available to the dog was reduced from 21 to 11 and then 9, 7, 5 and finally 3. Dogs were able to determine direction from 5 footsteps but not 3. It was calculated that it takes approximately 1-2 s for the odour information in footsteps to change to provide discernible information that can be used by dogs to determine direction. The process by which dogs may determine direction from odour cues is discussed.