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One of the main problems in selecting a control group in community based case-control studies is the availability of an appropriate source of the same geographical area. In Spain, the Municipal roll consists on a list of inhabitants being a potential source to obtain representative samples of general population. The structure of the Municipal roll is described with its contents in districts and census tracts. The strategy of the Municipal roll utilization in performing a childhood leukemia case-control study is also described. Its limitations are compared with those founded in the methods used currently in other countries like random digit dialing and birth certificates. Finally and given the current structure of the spanish Municipal roll, we discuss the usefulness of different approaches to obtain a random sample from this source and the problems which arose from using a method of matching by municipality, which is a cheap and quick procedure.

The results related to the controls response rate in a case-control study are presented. The study was designed to learn about the relationship between the occupational exposure to toxic products in the parents and the development of acute leukemia in childhood. 128 cases were enrolled in the study and each case was matched by sex and age (less than 15 years) to a control. Controls were selected from the Municipal roll of the same city or town in which the case has been living during the diagnosis. We contacted the control's mother by phone or by personal visit. The response rate was 47.9% and near to a half of non responses was due to errors in the addresses registered in the Municipal roll. The validity of this source of controls in Spain can be questioned in the light of the high percentage of errors. We also discuss the potential selection bias introduced in the study due to the way we selected controls.