This is a double blind study where patients will be involved and be under treatment for a period of 8 weeks. The initial evaluation will include rating scales measuring depression, anxiety, severity of illness, overall functioning, pregnancy test and clinical evaluations. Patients being treated with a SSRI or SNRI for at least 6 weeks, at therapeutic doses (see table 1.), who still have a HAM-D score of 18 or more, will be randomly assigned to treatment, either with Quetiapine, or Placebo. Patients in the active group will be titrated on a fixed schedule of 50 mg at night for 7 days, 100 mg at night for 7 days, then 200 mg at night for 7 days. After this the dose will be titrated upwards to a maximum of 600mg at night at the discretion of the investigator, using patient tolerance and response as guidelines over the duration of the trial. 200mg is in the range of the average dose in large trials of naturalistic clinical use of quetiapine and is actually much slower than the titration schedule on the package label. Patients that can not tolerate 200mg/day will be withdrawn from the study. Patients should have tried a minimum of 400mg /day before being withdrawn because of lack of efficacy. This dose will remain steady until week 8. Rating scales will be repeated every week during the first 2 weeks, and every 2 weeks up to the end of the study at week 8.