PURPOSE: To compare the efficacy of a 20-gauge and an 18-gauge needle in sono-guided percutaneous automated gun biopsy for establishing the specific diagnosis of renal parenchymal disease in pediatric kidneys. MATERIALS AND METHODS: In 60 pediatric patients with renal parenchymal diseases, percutaneous sono-guided gun biopsy was performed by an experienced radiologist. In two groups of 30 patients, regardless of their age, two needle passes were performed, using alternately an 18-gauge or a 20-gauge biopsy needle. The core of renal tissue thus obtained was examined with light, immunofluorescent or electron microscopy by the renal pathologist. The mean number of intact glomeruli of whole tissue core per biopsy, as seen on the light microscopy, and post-bioptic complications were compared between the two different needle size groups. RESULTS: The number (mean±1 standard deviation) ofglomeruli obtained per biopsy was 17±8 in the 18-gauge needle group, and 14±5 in the 20-gauge group. Between two groups, there was no major post-bioptic complication requiring specific treatment, nor a statistically significant difference in the frequency of minor complications. CONCLUSION: Even though more glomeruli were obtained with an 18-gauge needle, the number obtained with a 20-gauge needle also permitted adequate pathologic examination. Both an 18-gauge and a 20-gauge needle may thus be suitable for renal biopsy in pediatric patients.