Bardet-Biedl syndrome is an autosomal recessive disorder characterized by postaxial hexadactyly, obesity, mental retardation, pigmented retinopathy, hypogonadism, and renal disease. Morphological changes are present in all areas of the kidney, the renal medulla being the most frequently affected site. Cystic and dysplastic changes are prevalent. Seven children from five families were followed from birth through their 5th birthday. Serial renal sonography revealed a number of characteristic features. Bilateral renal enlargement and increased parenchymal echogenicity were present at birth. The usual corticomedullary differentiation was absent. Pyramids were either not seen or deformed. With high-resolution ultrasonography, small cysts were detected at the corticomedullary junction. After the 3rd month of life, there was a striking inversion of normal echogenicity, the inner medulla became more echogenic and was demarcated from the less-echogenic cortex. After 12 months, the kidney size regressed significantly. Fetal lobulation persisted in some patients. In conclusion, ultrasonography is a useful tool to evaluate the extent of renal lesions, but more importantly to differentiate bilateral polycystic kidney diseases in the newborn period.