To evaluate the clinical efficacy of Quetiapine 25 mg for the treatment of primary insomnia. A randomized, double-blind, placebo-controlled clinical trial was conducted. Patients with DSM-IV-TR defined primary insomnia were asked to record a sleep diary one week prior to treatment, followed by 2 weeks of nightly treatment with either Quetiapine 25 mg or placebo. The primary outcomes were total sleep time (TST), sleep latency (SL), daytime alertness and functioning and sleep satisfaction; side effects were recorded as secondary outcome. Data were collected between January 2007 and December 2007, at Srinagarind Hospital of Khon Kaen University. Thirteen patients completed the present study (mean age 45.95 years old; range 25-62). Quetiapine group increased mean TST by 124.92 minutes and 72.24 minutes in the placebo group. Mean SL was reduced by 96.16 minutes in the Quetiapine group and 23.72 minutes in the placebo group. Statistical significance was not reached between both groups. In the Quetiapine group two patients reported side effects of dry lips, dry tongue and morning drowsiness. The present study is the first study to evaluate the effect of Quetiapine in primary insomnia in a randomized controlled trial. Quetiapine at 25 mg at night did show a trend for improvement of TST and reduced SL in primary insomnia with few side effects but not reaching statistical significance. A study with a larger sample size is needed to demonstrate its efficacy.