In connection with treating juvenile bone cysts with intracavital corticosteroid injections, the interior cyst anatomy was analyzed at cystography in 13 children aged 4 to 15 years. Only 4 children had a true unicameral cyst; the others had 2 or more cysts or had compartments with free or restricted communication to the main cyst. Cysts and compartments which had not been reached by steroids may continue to grow. Optimum treatment may therefore require injections at different sites guided by cystography. Neither conventional criteria of cyst activity nor repeated bone scintigraphy 3 months after treatment could predict whether the treatment would lead to final healing.