In cryptorchid infants, significantly decreased mean levels of plasma testosterone and luteinizing hormone (LH) were found between the ages of 30 and 120 days. The levels of testosterone and LH were significantly correlated. No significant difference was found between infants with bilateral or unilateral cryptorchidism. After 120 days there was no longer any significant difference between cryptorchid infants and controls. No significant change in plasma follicle-stimulating hormone (FSH) was found. These data suggest that subnormal secretion of LH could be the primary abnormality in a proportion of boys with so-called common cryptorchidism. Our studies using LH-releasing hormone and human chorionic gonadotropin stimulation tests in older infants and children agree with the data obtained by measurement of basal plasma hormone levels during the first months of life. Anti-gonadotroph antibodies were found in the sera of approximately 50% of the cryptorchid children and infants studied, using an immunofluorescence technique. A study of 17 mothers and their infants gave concordant results in 16 pairs, 9 with and 7 without antibodies. This lead us to speculate on the possible role of maternal autoantibodies as a cause of partial gonadotrophin deficiency in the perinatal period and thus of testicular maldescent. As cryptorchidism is a syndrome, these findings do not mean that a similar mechanism is operative in all cases. However, these data do suggest that alternatives to the classical anatomical view of the descent and nondescent of the testes should be considered.