The investigators hope to learn if Letrozole is effective and safe in improving severe male infertility by increasing testosterone, decreasing oestradiol and stimulating sperm production thereby improving sperm motility (movement) and concentration. The study is being conducted because Letrozole is not yet proven to be a standard treatment in subjects with absent or very low sperm counts. The investigators are hoping to determine whether Letrozole is equal or superior to no treatment.

This is a pilot study with recruitment carried out in a single centre where participants will be assigned to receive Letrozole 2.5 mg once a day for 4 months. The following data will be collected: semen analyses, clinical history including smoking status, BMI, Y microdeletion, karyotype and FSH, LH, testosterone, E2, drug side effects and effects on libido. The following assessments will be performed before and 4 months after drug administration: sperm analyses, FSH, LH, E2 and testosterone measurements.

Prior to recruitment, the study participant would have undergone the routine investigations for evaluating azoospermia or severe oligozoospermia. These would include FSH, LH, testosterone, karyotyping, Y chromosome microdeletion genetic studies and at least 1 seminal analysis. Despite the availability of biochemical techniques to differentiate non-obstructive azoospermia (NOA) patients from obstructive azoospermia patients, fine needle aspiration (FNA) has been regarded as a highly informative and a minimally invasive method often routinely done in the investigation of azoospermia. All patients with NOA would have a FNA performed as part of their routine workup which would be done before recruitment and initiation of Letrozole. The safety of FNA use is also regarded as being supportive for the researchers against any possible patient lawsuits resulting from any impaired spermatogenesis induced by an off-label prescription. All oligozoospermic patients will be offered to have their sperm cryopreserved before drug administration. The investigations must have been performed within a 6 month time frame prior to recruitment.

There will be 3 clinic visits during the dosing period and 1 post dosing follow up visit. During the initial visit where recruitment into the study occurs, the participant will be prescribed the medication. One week later, a telephone interview will be conducted to review for side effects and arrange an early appointment at patient request. One month later, the participant will be reviewed in clinic and a side effect check list will be conducted. Telephone interviews will be conducted again at 2nd and 3rd month from administration of the drug. At 4th month from administration of the drug, the blood tests and semen analysis will be performed. It is likely that the participant will still be followed up for enrolment into IVF / ICSI programme should spontaneous pregnancy fail to occur. The window period for all clinic visits and telephone interviews is +/- 5 days.

Inclusion Criteria

• Patients with non-obstructive azoospermia (absence of sperm in the pellets of two centrifuged semen samples collected 30-60 days apart)
• Patients with non-obstructive azoospermia who do not yield spermatozoa with fine needle aspiration (FNA)
• Patients with severe oligozoospermia (presence of sperm concentration less than 5 million per ml in both semen samples collected 30-60 days apart)
• Normal sperm appearance, consistency, liquefaction, volume and pH
• Patient must not possess any chromosomal aberrations

Exclusion Criteria

• Possible etiology of infertility present
• Seminal white blood cell concentration more than 10 million per ml
• Positive seminal culture analysis
• Positive urethral swab for chlamydia test
• Smoker
• Drug or alcohol abuse
• Ongoing medical treatment (gonadotropins, anabolic steroids, cancer chemotherapy, non-steroidal anti-inflammatory drugs, previous cancer radiotherapy or chemotherapy)
• Palpable varicocele
• X-ray exposure in the previous 8 months
• Y chromosome microdeletion
• Karyotype abnormalities (Klinefelter syndrome)