Using IS 6110 -restriction fragment length polymorphism (RFLP) and spoligotyping, genetic variations of 83 Mycobacterium tuberculosis strains isolated from tuberculosis patients from two wards in a hospital in Delhi and a rural chest clinic near Delhi were analysed. The vast majority of the isolates (75%) were closely related and this novel genogroup was designated the 'Delhi type'. Both drug-sensitive and drug-resistant strains were found among strains of this genogroup. A minority of the strains harboured a single IS 6110 copy and only one strain belonged to the Beijing genotype, a genotype that is predominant in other parts of Asia. A comparison of the RFLP and spoligotype with existing data suggests that the predominance of Delhi genogroup is geographically limited to the Indian subcontinent and perhaps to specific regions in India. Despite the high prevalence of the M. tuberculosis strains of the Delhi type, the strains could easily be discriminated due to polymorphisms in the IS 6110 patterns. Future studies may disclose the genetic characteristics of strains belonging to the Delhi genotype, analogous to the recently observed virulence among the Beijing genogroup.