1. The rare occurrence is reported of identical twins concordant for severe obsessive-compulsive neurosis. In both the onset occurred at age 10 and the disorder consisted of fear of contamination by dogs, which produced compulsive washing and marked restriction in daily activities. The similar content of the obsessions was presumably derived from social interaction, but no amelioration of symptoms occurred in either twin during eighteen months' separation. The dominant twin had the more severe symptoms but instigated referral of his brother for treatment.
2. This brother was treated by desensitization with but marginal improvement. Difficulties included very slow progress of desensitization, lack of transfer of improvement from one situation to another, and resistance by the patient to changing his complex habits.
3. At one stage in desensitization both clinical and physiological dissociation occurred between anxiety and compulsions, suggesting that compulsive behaviour is not always maintained solely by its anxiety-reducing properties.
4. Further treatment by restriction of washing and aversion relief produced slight improvement in the compulsive washing.