A woman born in 1920 has suffered from a chronic destructive lung disease since 1972, with development of a severe combined restrictive and obstructive ventilatory defect. Large quantities of acid-fast microorganisms have been repeatedly observed in her sputum. Multiple courses of antimycobacterial treatment did not stop the progression of the disease. The mycobacterium involved was first identified as Mycobacterium gordonae, and later as Mycobacterium scrofulaceum. Analysis of part of the amplified gene of the 16S rRNA, however, revealed that its sequence differed from that of any established mycobacterial species, although it was observed once before in a German lymph node isolate, for which the name "Mycobacterium interjectum" has been proposed. Retrospective analysis confirmed the presence of this sequence in frozen samples which had been provided by the patient in 1983, 1985, 1989, 1990, and 1993. Our case confirms the value of amplification and sequencing of mycobacterial 16S rRNA for classifying mycobacteria, and suggests that "Mycobacterium interjectum" may be involved in cases of chronic destructive lung disease.