Table 3.

Matched analysis of the 89 affected sibling pairs discordant for smoking at diagnosis

	Smokers
	CD sibling	UC sibling
Non-smokers
CD sibling	42	2
UC sibling	21	24

23 pairs were discordant for their diagnoses. In 21 of these pairs, the smoker had been diagnosed as having Crohn's disease (CD), and the non-smoker ulcerative colitis (UC) (McNemaras test with Yates' continuity correction p<0.0001, odds ratio 10.5, 95% confidence limits 2.6–92; Fisher-Irwin two tailed probability test p<10⁻⁸). There were only two affected sib pairs in which the smoker had developed UC and the non-smoker CD.