DISEASE CHARACTERISTICS: Histologically confirmed small cell lung cancer for which no potentially curative therapy exists Confirmation at relapse required only if sole relapse site is within previous radiation port No mixed histology Bidimensionally measurable disease by CT scan At least 1 measurable lesion at least 2 cm At least a partial response to front line chemotherapy Single regimen or alternating regimen allowed No initial course exceeding 8 courses or lasting more than 6 months No uncontrolled, untreated brain metastases No extensive liver metastases such that greater than 50% of liver parenchyma is replaced with metastatic disease