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To find out whether keeping birds in the home is an independent risk factor for lung cancer a case-control study was carried out in four main hospitals in The Hague, The Netherlands. Forty nine patients under 65 years of age with lung cancer each were matched for age and sex with two control subjects who attended the same general practice. Data were collected on social class, cigarette smoking, intake of beta carotene and vitamin C, and alcohol consumption. It was found that smoking, birdkeeping, and a low intake of vitamin C were significantly and independently related to the incidence of lung cancer. The odds ratio for lung cancer among people who keep birds as pets was estimated to be 6.7 after adjusting for smoking and vitamin C intake. The results of this study suggest that keeping pet birds is an independent risk factor for lung cancer.