In 2 laying hen flocks (housed in aviary systems) depressed performance, increased mortality and cannibalism were observed. Specific infectious diseases were excluded. The hygienic quality of the diet, however, based on self produced feed ingredients, was impaired. High concentrations of bacteria, moulds, and yeasts were found in the complete diet and also in the ingredients. These problems were eliminated by providing the birds with good quality feed ingredients. In a diagnostic feeding trial with a change in housing conditions hens showed a good performance. The poor feed quality, as demonstrated in this report, is regarded to be one of several factors, which have contributed to the problems observed in these flocks. Other negative environmental factors such as housing have to be eliminated.