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The annual mean diesel particulate concentrations in 1979 were estimated for four urban areas -- New York, Los Angeles, Washington, DC and Denver. These values were then compared with projections for the year 2000 under three different NOx emission scenarios. The particulate emission rates chosen for the three scenarios are best engineering estimates which include the implementation of advanced technology. It was found that diesel particulate concentrations remained about the same as the 1979 levels for the high NOx scenario, but doubled or more than doubled for the low NOx scenario. The low NOx scenario will also lead to a visibility deterioration of 5% or more compared to 1979 visibility. For all scenarios, the contributions of buses and heavy duty trucks to diesel particulate concentrations are comparable for all cities except Los Angeles, with a combined contribution of 65 to 85%.