The primary research objective of this observational study in pretreated patients with NSCLC of Stage IIIa/b or Stage IV is to evaluate patients' Karnofsky Index (KI) after the second cycle of second-line treatment with single agent pemetrexed in a real life setting, and to evaluate factors potentially influencing performance status in terms of KI, i.e. frequency of use of concomitant medications, in particular analgesics, folic acid, Vitamin B12, antiemetics, and medications against diarrhea; as documented in patient diary (days of use per week) and by the physicians.

Inclusion Criteria:

• Are 18 years of age or older
• Have NSCLC, Stage IIIa/b or IV, that is not amenable to curative therapy
• Have had one prior chemotherapy regimen with cytotoxic agents for the treatment of NSCLC (patients with adjuvant and neoadjuvant regimens are also eligible. However, patients with one prior cytotoxic chemotherapy followed by a targeted agent such as erlotinib are not eligible because the targeted agent would be regarded as second-line treatment)
• Are initiating second-line of treatment for NSCLC with pemetrexed
• Have been fully informed and have given written consent for the use of their data.

Exclusion Criteria:

• Have received any prior treatment with pemetrexed
• Are participating in a study including administration of any investigational drug or procedure at entry into this study

Patients with NSCLC Stage IIIa/b or IV who have previously been treated with chemotherapy and are starting second-line treatment with single-agent pemetrexed are eligible for this observational study. Patients must have received one prior chemotherapy regimen. Patients who received prior neoadjuvant or adjuvant therapy are also eligible. Both, in- and outpatients are eligible.