In nine neonates, spontaneous regression of a multicystic dysplastic kidney (MCDK) was witnessed by means of repeated ultrasound (US) examinations. In three of these patients, the diagnosis was made in utero. Follow-up examinations at the ages of 3, 5, and 32 weeks post-partum showed what would have been called unilateral agenesis of the affected side if no fetal US study had been done. In the remaining neonates, the diagnosis was made postnatally, and marked reduction in size or complete disappearance of the MCDK was observed on serial US examinations. Three neonates underwent surgical exploration. No trace of a kidney, renal artery, or ureter was found in two. A small MCDK was removed in the third patient. US reveals new features of the natural history of MCDKs. Because malignant transformation of an MCDK is rare and because US provides a means of serial assessment, the authors believe a more conservative, nonsurgical approach is appropriate and recommend an observation time of 1 year before deciding on surgical intervention, unless other problems necessitate surgical removal of the MCDK.