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A cohort of some 11 000 men born and employed for at least one month in the chrysotile mines and mills of Quebec Of the 33 deaths from mesothelioma in the cohort to date, 28 were in miners and millers and five were in employees of a small asbestos products factory where commercial amphiboles had also been used. Preliminary analysis also suggests that the risk of mesothelioma was higher in the mines and mills at Thetford Mines than in those at Asbestos. More detailed studies of these differences and of exposure-response relations for lung cancer are under way.

SUMMARY In a case-referent study of matched pairs with a single dichotomous exposure variable, the relative risk R is estimated from the "discordant pairs," r and s, by R = ris. In this paper the exact (1 -a) confidence limits, RL and RU, have been simplified, and a new exact test of the null hypothesis that R = 1 has been derived. The exact methods depend on the F distribution; they are simple to calculate and all have been fully validated. Even after simplification, methods for obtaining approximations to RL and RU remain more unwieldy than the exact methods, and an evaluation shows that these approximations may occasionally be far from the truth. It is argued that there is no excuse for not using the exact methods. (2) where the degrees of freedom for FL are 2(s + 1) and 2r. The expression (2) comes from B&D's (5.7) and (5.9), remembering that their n10 and n0o are Armitage's r and s, so that their Fa,/2(2no, + 2, 2n10) is my FL. The upper (1 -a) confidence limit is Ru= (r + 1)Fu/s (3) with 2(r + 1) and 2s degrees of freedom for Fu at the 4a probability level. Derivation of (3)