The cockroach allergen (Bla g 1) content was determined in the floor dust of 46 homes with recent cockroach extermination in Amsterdam, The Netherlands. IgE antibodies to Blattella germanica, house-dust mite, cat dander, dog dander, and a mixture of molds were determined in venous blood samples of 46 children (4-12 years) and one of their biologic parents (24-54 years). Specific IgE to cockroach was also determined in a sample of the general population studied in a previous case-control study, one group (n = 20) with three groups (n =76) without history of cockroach infestation of the home. Cockroach allergen was detected in floor dust from 44% of the homes, with levels up to 3899 ng Bla g 1/g. Seven of the 46 adults and only one of the 46 children studied had positive RAST to cockroach. Geometric mean cockroach allergen concentrations in living room and master bedroom of sensitized adults were similar to those of nonsensitized adults. In the groups of children without a history of cockroach infestation of the home, positive RAST against cockroach was observed in 16% of the children with respiratory symptoms, in 4% of the children without respiratory symptoms, and in 48% of the children with two or more positive RAST to other allergens. Of the 18 children with positive RAST against cockroach, only one had a history of cockroach infestation of the home and 16 (89%) had also positive RAST against house-dust mite.