BACKGROUND: Adult-onset Lhermitte-Duclos disease (LDD) and Cowden syndrome (CS) are considered a single phakomatosis that belongs to PTEN hamartoma tumor syndrome (PHTS) now. There is still controversy regarding the diagnosis and treatment. The authors describe the clinical features of LDD and CS with long-term follow up. METHODS: From January 2001 to January 2017, 18 patients were admitted to the neurosurgery department of Beijing Tiantan Hospital. The authors analyzed themedical records of each patient and followed every case. RESULTS: Seventeen of 18 patients underwent surgery to remove the tumor. The results of pathologic analysis revealed LDD. There was obvious enhancement on magnetic resonance imaging (MRI) in 2 patients who received gamma knife and radiotherapy before surgery. During surgery, it is difficult to determine the exact margin. Tumors were removed gross totally in 9 patients, partially in 6 patients, and only subtotally in 2 patients. CS was diagnosed in 11 patients. Two patients received DNA analysis, revealing heterozygous mutation of exon 5 in an 11-year-old girl. There was no recurrence of the tumor during follow-up. CONCLUSIONS: LDD has the unique appearance on T2-weighted MRI. The most difficult aspect of surgery is determining the actual margins of the tumor. Total resection is difficult in some patients. There was no tumor recurrence after long-term follow-up in our case series. For pediatric LDD patients, DNA analysis should be performed to rule out CS.