Inclusion Criteria:

- Informed consent signature.

- Men aged 18-50 years and women aged 18-45 years.

- Patients diagnosed of schizophrenia or bipolar disorder according to DSM-IV and on
ambulatory psychiatric follow-up.

- Treated with one or more antipsychotics other than quetiapine for at least 8 weeks
prior to inclusion in the study.

- Prolactin serum levels over 20 ug/L.

- That, in the opinion of the clinician, the origin of the hyperprolactinemia is the
antipsychotic treatment administered previously to the patient.

- Switching from previous antipsychotic treatment to quetiapine following the clinical
criterion of the investigator in response to hyperprolactinemia in a period less than
5 days prior to inclusion in the study.

- Women with child-bearing potential using an effective contraceptive method and with a
negative result in the pregnancy HCG test at the time of inclusion.

- Be able to understand and meet the study requirements.

Exclusion Criteria:

- Pregnant or nursing women.

- Mental retardation.

- Dependence or abuse of substances on inclusion according to DSM-IV criteria.

- Patients that, in the investigator's opinion, are at a high risk of suicide or mean a
risk of aggression to others.

- Treated with any of the following drugs that can modify PRL levels on inclusion and
during the study: antipsychotics, except for quetiapine, antidepressants -except for
mirtazapine-, hormone therapy, spermicides, antiparkinson drugs or dopaminergic
agonists, metoclopramide, domperidone, cimetidine or ranitidine, verapamil,
enalapril, alpha-methyldopa, reserpine, morphine and other opioid derivatives,
anti-retrovirals (protease inhibitors), vitamin D or any complex containing it. These
drugs are excluded for their known potential effect on prolactin serum levels, sexual
hormones and/or cortisol.

- Administration of an antipsychotic depot injection in one of the usual administration
intervals of the depot (e.g., 3-4 weeks) prior to inclusion.

- Be treated with any of the following P450-3A4 cytochrome inhibitors in the 14 days
prior to inclusion, including: ketoconazole, itraconazole, fluconazole, erythromycin,
clarithromycin, fluvoxamine, indinavir, nelfinavir, ritonavir and saquinavir.

- Be treated with any of the following P450-3A4 cytochrome inducers in the 14 days
prior to inclusion, including: phenytoin, carbamazepine, barbiturates, rifampicin,
St. John's wart, and glucocorticoids.

- Any contraindication to the use of quetiapine fumarate in the investigator's opinion
(including lack of response to it in previous treatment attempts) (applies also to
any other treatment to be used in the study -comparative agents-).

- Suffer any medical condition that can effect the absorption, distribution, metabolism
or excretion of the study treatment(s).

- Suffer any medical condition in decompensation or not receiving inappropriate
treatment for it in the investigator's opinion (e.g., diabetes, angina pectoris,
hypertension...) and can affect psychotic symptoms and/or levels of prolactin, sexual
hormones and/or cortisol.

- Suffering unstable diabetes.

- Absolute neutrophil count £1.5 x 109 per litre.

- Non-compliance with the study plan.

- Participation in a clinical trial in the four weeks prior to inclusion in the study.