DISEASE CHARACTERISTICS: Histologically or cytologically proven small cell lung cancer Complete response (CR) or partial response (PR) following first line chemotherapy required Chest x-ray showing CR or PR required. No documented prior brain metastases

PATIENT CHARACTERISTICS: Age: 16 and over Performance status: ECOG 0-2 Life expectancy: At least 12 weeks Hematopoietic: Not specified Hepatic: Not specified Renal: Not specified Other: Not pregnant or nursing Effective contraception use by men or women of reproductive potential No prior malignancies within 5 years except: Adequately treated nonmelanomatous skin cancer Adequately treated carcinoma in situ of the cervix No other concurrent malignancies No prior diagnosis of breast cancer, melanoma, or hypernephroma No major medical illness that would preclude prolonged administration of marimastat or required follow up No active peptic ulceration or symptoms suggestive of this diagnosis No grade 3 or 4 musculoskeletal disorders

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: One prior induction combination chemotherapy regimen required Must be completed prior to randomization Hematologically recovered before randomization Minimum of 4 cycles required No change in regimen due to progression No chemotherapy within 28 days prior to randomization if thoracic radiation is given prior to or concurrent with chemotherapy No prior marimastat Endocrine therapy: Not specified Radiotherapy: Prior radiotherapy allowed Must be completed prior to randomization Last dose of radiation treatment must be within 7-14 days prior to randomization if thoracic radiation and/or prophylactic cranial irradiation is given after completion of chemotherapy If severe esophagitis precludes administration of oral medication, randomization may be within 21 days after radiation therapy Surgery: No surgery within 2 weeks prior to randomization Prior complete resection of tumor allowed Other: No other investigational agents within 4 weeks prior to study, and none planned No concurrent coumarin anticoagulants and no coumarin anticoagulants within 4 weeks prior to randomization No concurrent antitumor treatment