Maintenance ZD1839 (IRESSA®) Following Completion of Front Line, Platinum-Based, Double Chemotherapy in Subjects With Advanced or Metastatic Non-Small Cell Lung Cancer

Inclusion Criteria: * Must sign Informed Consent * Females and males aged 18 years and over. * Confirmed stage IIIB (T4-pleural effusion) and IV NSCLC. * Either complete response, partial response, or stable disease following at least 4 cycles (3 in the setting of intolerable toxicity) but no more than 6 cycles of definitive front line, platinum-based, doublet chemotherapy. * No prior EGFR therapy * No newly diagnosed intracerebral metastases while receiving or after completing chemotherapy * At least 3 weeks (21 days) but no more than 4 weeks (28 days) since last dose of chemotherapy * Must be completely healed from previous major oncologic surgery * Life expectancy of ≥ 8 weeks. Exclusion Criteria: * Known severe hypersensitivity to ZD1839 or any of the excipients of these products. * Any evidence of clinically active interstitial lung disease (subjects with chronic, stable, radiographic changes who are asymptomatic need not be excluded). * Other co-existing malignancies or malignancies diagnosed within the last 5 years with the exception of basal cell carcinoma or cervical cancer in situ. * Any unresolved chronic toxicity greater than common toxicity criteria (CTC) grade 2 from previous anticancer therapy excluding peripheral neuropathy or alopecia. * Evidence of any significant clinical disorder or laboratory finding that makes it undesirable for the subject to participate in the study. * Pregnancy or breast feeding (women of child bearing potential). Women of childbearing potential must practice acceptable methods of birth control to prevent pregnancy. * Treatment with a non-approved or investigational drug within 30 days before Day 1 of study treatment. * Signs of neurological symptoms consistent with new onset spinal cord compression or CNS metastases. * Treatment with any systemic anticancer therapies other than the prescribed protocol chemotherapy regimen (refer to Inclusion criterion -). Exception: Palliative radiotherapy for symptom relief of lesions present at diagnosis will be allowed; however, this radiotherapy must occur prior to completion of pre-study doublet chemotherapy. * Males must also be willing to practice acceptable methods of birth control while taking the drug to prevent pregnancy of a partner.