Allergy to pollen of Cupressaceae has been linked to pollens of Cupressus, Juniper and Cryptomeria. The authors report 2 cases of rhinitis and conjunctivitis induced by thuja, another member of the Cupressaceae family. Monosensitization to thuja pollen has been identified as the causal agent: (1) of a long-standing springtime rhinitis in 1 patient with negative skin tests and specific IgE titers to the main inhalants (specificity of the prick test and nasal provocation with the thuja extract was confirmed by a positive RAST) and (2) in a 2nd patient without former history of allergy, who consulted for conjunctivitis following acquisition of a dog; the standard battery of skin tests, and Phadiotop were all negative. After controls, only the thuja extract gave significant skin test response. The level of total IgE was low, and RAST was negative. Sensitization and pollen provocation were produced by the intermediary of the dog, carrying thuja pollens on its fur. The immunoprint and the crossed radioimmunoelectrophoresis revealed common antigenicity between cypress and thuja extracts.