Context. Phenotyping of diseases is essential for genetic studies. In common diseases like psoriasis, the co-operation of family practitioners is helpful to confirm the phenotype of the patients. A genetic study was initiated in France by Généthon in 1996 in order to localise and identify genes for susceptibility to psoriasis. We used an unusual means to confirm psoriasis diagnosis: letters sent to patients' practitioners. Objective. To evaluate the efficiency of this means, and analyse the factors influencing the response rate of patients' physicians to a letter designed to confirm a diagnosis of psoriasis. Design. In the cases of putative psoriasis diagnosis (evaluated by systematic phone calls to the patients by a dermatologist), and with the agreement of the patients, a letter was sent to the patient's referent physician. We evaluated response rate to the letter. Results. 533 letters concerning 762 patients were sent to 456 physicians. The global response rate was 56.5%. The characteristics which modified the response rate were: the quality of the letter sent and some physicians' characteristics: dermatologists versus GPs, hospital versus private practice, and size of the town in which the physician had his practice. Conclusions. The use of letters to physicians as a means of phenotyping patients' diseases is an efficient method of confirming clinical diagnosis and should be used more frequently to confirm clinical diagnosis in large genetic studies. Some parameters such as ease of reply to the letter, whether the physician was a GP or a specialist, and the type of practice increased the response rate.