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The use of lead compounds, when added to gasolines to enhance their antiknock properties, has resulted in more efficient combustion in high-compression engines, but their use has also caused lead deposits in the combustion chamber. To assess how lead antiknock compounds in gasolines influence combustion chamber deposits and exhaust emission levels, an intensive state-of-the-art review has been made, and data submitted by 18 different companies have been analyzed. These studies ranged in scope from single-cylinder engine investigations to a 122-car consumer test program. Based on this review, it was concluded that cars operated on leaded gasolines have higher equilibrium hydrocarbon emissions than those operated on unleaded gasolines, and that mileage accumulation conditions exert a major influence on the magnitude of the hydrocarbon net lead effect. The presence of lead in gasoline has no effect on carbon monoxide emission levels.