The value of the ultrastructural study of the renal biopsy was investigated in a series of pediatric patients with nephrotic syndrome. Forty-eight cases of renal biopsies with clinical data were reviewed and divided into diagnostic groups. The contribution of electron microscopy to the final diagnosis was graded as essential-diagnosis could not be reached without it; supportive-it increased the level of confidence in the final diagnosis; and noncontributory. In this series of renal biopsies from 48 children with nephrotic syndrome resistant or nonresponsive to therapy, the most frequent diagnosis was minimal change disease, present in 42% of the patients. The contribution of the electron microscopic study to the final diagnosis was essential in 73% of the series, and was supportive in a further 27%. Therefore, it is concluded that the ultrastructural study was an essential component in the study of the renal biopsy in children with nephrotic syndrome, suggesting that electron microscopy needs to continue to be performed for all these patients.