For more than 20 years there has been a coherent programme of product modification in the UK, involving the introduction of low tar brands of cigarettes and the gradual reduction in average tar yields over the whole range of manufactured cigarettes. The sales-weighted average tar yield has declined from 20.8 mg/cigarette in 1972 to 11.0 mg/cigarette in 1993. To some extent potential benefits to established smokers have been offset by their tendency to 'compensate' for reduced nicotine yields. Investigating such aspects has formed one part of a wide-ranging research programme to monitor effects of modified products on health. Collectively the studies show benefits in terms of smoke intake and health outcome related to reduced tar cigarettes, but the success achieved in reducing average tar yields and narrowing the range has limited opportunities to detect differential effects.