The purposes of this study are to determine 1) the safety of pemetrexed and any side effects that might be associated with it 2) whether pemetrexed can help patients with small cell lung cancer live longer 3) whether pemetrexed can make tumors smaller or disappear and for how long and 4) to see if patients feel better while taking pemetrexed

Inclusion Criteria:

• Diagnosis of small cell lung cancer
• One prior treatment with chemotherapy or immunotherapy with greater than 3 weeks since the prior treatment
• At least 18 years of age
• Estimated life expectancy of at least 12 weeks
• Ambulatory and capable of self-care(eg, up and about greater than 50% of waking hours)

Exclusion Criteria:

• Have received treatment within the last 30 days with a drug that has not received FDA approval for any indication
• Less than 2 weeks from radiation therapy
• Other serious illness that would compromise the safety of the patient
• Most second primary malignancies treated less than 5 years previously