第二大臼歯の萌出障害は比較的稀であるが，患児は13歳10か月時，上下顎左右側第二大臼歯部の4か所に嚢胞様病変を認め，そのうち3歯の第二大臼歯に萌出遅延を生じていた。特に下顎左側第二大臼歯はほぼ水平に近心傾斜し，歯冠は嚢胞腔内に含まれる形で埋伏していた。水平埋伏した下顎第二大臼歯は抜歯となることが多いが，本症例では，下顎左側第二大臼歯部の嚢胞に対して開窓を行い，嚢胞治癒後に同歯の萌出誘導を行って，歯列上に配列することができた。すでに萌出していた下顎右側第二大臼歯の嚢胞様病変についても，開窓を行い良好な結果を得た。上顎両側第二大臼歯は未萌出であり，歯冠周囲の歯小嚢の拡大像がみられた。上顎右側第二大臼歯は14歳9か月時に自然萌出したが，左側第二大臼歯はびまん性の腫脹が生じたため，16歳1か月時に開窓を行い，歯列内に配列させることができた。下顎左側第二大臼歯の萌出障害の直接的原因としては，嚢胞または歯胚の萌出方向の異常が考えられた。本症例は，類似した嚢胞様病変が上下顎の4か所に生じたが，多発嚢胞を生じた原因は明らかではない。多発嚢胞は基底細胞母斑症候群に多く発症するといわれているが，本症例では同症候群との関連はないと考えられた。しかし，基底細胞母斑が出現するのは思春期以降から35歳ごろであるといわれており，今後も経過観察を行う必要があると思われた。Eruption disturbance of the second permanent molar is rare in clinical pediatric dentistry． In the present case， the patient had four cystic lesions adjacent to the upper and the lower bilateral permanent second molars at 13 years 10 months of age． Out of them， three molars were delayed in erupting． In the mandible， the left second permanent molar impacted horizontally and the crown was included in the cystic lesion． Fourteen months after surgical exposure of the impacted molar and the lesion， the radiolucent area of the cyst was abolished and replaced by new bone． Thereafter， the second molar was guided for eruption and aligned properly within the dentition． On the other side， the cystic legion adjacent to the erupted right second permanent molar was surgically exposed and healed 3 months later． In the maxilla， cystic legions were detected bilaterally over the crowns of the unerupted second molars． Although the right second molar erupted spontaneously at the age of 14 years 9 months， the left one was surgically exposed at the age of 16 years 1 month because of non-eruption and swelling． The main causes for the eruption disturbance of the three permanent second molars were presumed to be pressure of the cystic lesion and/or abnormal eruptive direction of the tooth germ． It seems to be quite rare for the four cystic lesions to occur adjacent to all of the second molars． Although the present patient is not suspected to have a basal cell nevus syndrome， careful observation will be important in her case to be carried out from puberty to around 35 years of age when the disease often appears.