Sixty-two dogs with problems when travelling in the car took part in a non-blinded study aimed at differentiating groups of dogs on the basis of the pattern of signs shown by individual dogs and their response to treatment with a collar impregnated with dog-appeasing pheromone (dap) for six weeks and general behavioural advice. The dogs were taken out in the owner's car at least twice weekly for nine weeks, and their behaviour was assessed every three weeks to determine the frequency of 21 behavioural signs. On the basis of these signs and by using principal components analysis, the dogs were grouped into five well defined groups, defined as ‘excitable’, ‘nausea’, ‘tense’, ‘attention-seeking’ and ‘elimination’. For the purpose of statistical analysis, the attention-seeking and elimination groups were combined when assessing the effect of the treatment. All the groups showed some statistically significant improvements after treatment, but their responses were not uniform. The greatest perceived improvement was among the nausea group and the least was among the excitable group. In the groups for which there were sufficient data for analysis, there was little evidence of a relapse in most of the signs in the three to five months after the collar had been removed.