Hi, Renal agenesis (non-development of kidney) is an uncommon occurence. Usually, the kidney is ectopic (not in normal position) and needs to be searched for in the rest of the abdomen. More common is dilatation of the renal pelvis due to some obstruction at the pelviureteric junction which may appear as a cyst when very severe. True renal cysts may also occur. Rarely, the kidney undergoes shrinkage due to chronic disease (eg: renal artery stenosis) and becomes so small that it may not be seen on USG. Finally, a very unlikely scenario is the presence of a worrisome renal cystic lesion. A dilated bowel may appear cystic but is usually very easy to differentiate from a simple renal cyst due to a thick, stratified wall and presence of peristalsis (contraction of bowel). Although these are all various causes in which a kidney may not be visualized with presence of a cyst in place, most often none of these conditions require any urgent treatment, especially if the child is slightly older (above 5-10 years). Also, since the other kidney is normal, survival is not a problem. A repeat USG may solve the problem by detecting presence of renal tissue. If confusion still persists, a cross sectional study (like CT / MRI) maybe of help. Finally, it might be helpful to get her BP (blood pressure) checked (unlikely possibility of renal artery stenosis) and if raised, a medical opinion maybe required. Hope this helps, Sincerely Vamshi