EDITOR, - Richard Lilford and colleagues conclude that subfertility in men is familial and that their observations are consistent with a genetic cause.1 Measurement error and confounding may provide alternative explanations.

In case-control studies it is essential that the data collected from cases and controls should be of comparable accuracy - a requirement not met by this study, in which infertility in the brothers of cases and controls was assessed on hearsay evidence. The reasons for childlessness are more likely to be explored when two brothers are childless than when one brother is fertile and the other childless. Subfertile men may actively seek the explanation of their brother's childlessness in an attempt to explain their own subfertility. Cases and controls may interpret unexplained childlessness from their own experience as involuntary or voluntary. Additional bias may be introduced if the collection of information was not blinded. The use of reported childlessness also assumes that all such childlessness is due to male subfertility, the partner's contribution being ignored. Misclassification could easily explain these results.

We agree that the difference in …