Inclusion Criteria:

• Histologic or cytologic diagnosis of Small Cell Lung Cancer (SCL).
• Functional stage from 0 to 2 of the ECOG functional scale
• No previous systemic chemotherapy, immunotherapy or biologic therapy for SCLC.
• Previous bone marrow radiotherapy less than 25% is allowed.
• There must be at least one measurable lesion that complies with the solid tumor response evaluation criteria.
• Appropriate organic function.
• Life expectancy estimated at 12 weeks minimum.
• Females must be surgically sterile, postmenopausal or follow approved medical contraceptive methods during the treatment period and 6 months afterwards. Males must be surgically sterile or use a contraceptive method during the treatment period and during 6 months after treatment.
• The patient must be compliant and located close to the trial area for appropriate follow-up.
• The patient or his/her legal representative must sign an informed consent document.
• Patients must be at least 18 years of age.

Exclusion Criteria:

• Having received treatment for the last 30 days with a drug that has not obtained regulatory approval.
• Having participated in a previous pemetrexed trial.
• Mixed histologic diagnosis of SCLC and NSCLC.
• Concurrent illness.
• Having an active infection.
• Severe cardiac disease.
• Having received recently or concurrently a vaccine against yellow fever.
• Having suffered a previous malignant process other than SCLC.
• Central nervous system (CNS) metastases require concurrent corticoid therapy. Treated and stable CNS metastases are allowed.
• Clinically relevant fluid accumulation in the third space.
• Significant weight loss (greater than or equal to 10%) within 6 weeks prior to trial inclusion.
• Concurrent administration of any other anti-tumor treatment.
• Severe renal failure.
• Unable to discontinue administration of non-steroidal anti-inflammatory (NSAIDS) agents.
• Inability or unwillingness to take folic acid and vitamin B12 supplements.
• Inability to take corticoids.