Twenty-seven opportunistic deep-seated fungus infections in Turkish children were studied. The incidence in autopsied cases was 3.1 %. Five types of fungus infections were found. They were aspergillosis, candidiasis, cryptococcosis, mucormycosis and penicillosis. The most frequent infection was candidiasis which occurred mostly in infants and young children with gastroenteritis and malnutrition. Thymic dysplasia, as a direct evidence of deficiency of cellular immunity, was seen only in candidiasis. Thymic atrophy seen in cases of malnutrition probably played a similar role later in the course of the disease. Dehydration and acidosis were frequent and trauma was also significant in candidiasis and, in this type of infection, the gastro-intestinal tract was usually involved. In contrast, candidiasis, aspergillosis, and mucormycosis were seen in older children (mean age 9 years old) and were frequent in lympho-reticular malignancy, especially leukemia; in the majority of the cases the lungs were involved. A variety of other less-frequently encountered predisposing diseases such as renal and hepatic failure, congenital adrenal hyperplasia and malabsorption syndrome with A avitaminosis were found.