A series of 3-substituted 5-hydroxy-5-trifluoro[chloro]methyl-1H-1-isonicotinoyl-4,5-dihydropyrazoles (2a-i) were synthesised by the cyclocondensation reaction of 4-methoxy-1,1,1-trifluoro[chloro]-4-(substituted)-alk-3-en-2-ones (1a-i) and isoniazid (INH). Their in vitro antimicrobial activity was tested against INH-susceptible Mycobacterium tuberculosis H37Rv, INH-resistant clinical M. tuberculosis isolates and non-tuberculous mycobacteria. Amongst the synthesised compounds, 5-hydroxy-5-trifluoromethyl-4,5-dihydro-1H-1-(isonicotinoyl)-pyrazole (2a) and 5-hydroxy-3-(4-methylphenyl)-5-trifluoromethyl-4,5-dihydro-1H-1-(isonicotinoyl) pyrazole (2d) were found to be the two most active agents against susceptible M. tuberculosis and several INH-resistant strains. The compound 3-(2-furyl)-5-hydroxy-5-trifluoromethyl-4,5-dihydro-1H-1-(isonicotinoyl)pyrazole (2f) was active against all the INH-resistant strains regardless of the genetic background at concentrations two- to four-fold its minimum inhibitory concentration against M. tuberculosis H37Rv. These compounds were inhibitors of mycolic acid biosynthesis, in agreement with the utilisation of the INH scaffold for their design. Interestingly, the most active compound against M. tuberculosis, 5-hydroxy-5-trifluoromethyl-4,5-dihydro-1H-1-(isonicotinoyl)-pyrazole (2a), was even more potent than INH against non-tuberculous mycobacteria.