High Concentrations of lead, one of the most highly toxic heavy metals, in ambient air are commonly found in major urban areas where motor vehicle density and leaded gasoline consumption are high. Lead, in the form of tetraethyl lead or tetramethyl lead, is normally added to the gasoline to boostg up the octane level of the gasoline to prevent knocking of the engine and is subsequently emitted with the exhaust gas through the tail pipe into the ambient air as lead oxide and lead bromide particles having a mass median equivalent diameter of less than 1 mm. Exposure to high levels of lead could result in lead poisoning which includes the most critical effects on heam biosynthesis, erythropoiesis, the nervous system, and blood pressure, Children up to 6 year of age are a population at increased risk for lead exposure as well as for adverse health effects. Thailand has started phasing out lead from the gasoline since 1989. Since then, it took Thailand only 6 years to completely phase out lead from the gasoline . Lead content in the gasoline was gradually reduced from 0.45 gm/liter to 0.4 gm/liter in 1989 and to 0.15 gm/liter in 1992 . Concurrently, premium unleaded gasoline was first introduced in Thailand in 1991 and regular unleaded gasoline was mandated in July of 1994. Finally since January 1 of 1996, leaded gasoline has been completely banned in Thailand Consequently, ambient air lead concentrations in both general and roadside areas of Bangkok have drastically decreased starting from 1991 coinciding with the implementation of the program to phase out lead but in contrast with the increase in gasoline consumption. The maximum roadside 24-hr average lead concentration of 7.56 mm/m3 and the annual average concentration of 1.52 mm/m3 found in 1991 was reduced to 0.52 mm/m3 and 0.1 mm/m3 in 1996 ,respectively, The same trend is also found in the general areas where the maximum 24-hr and annual average lead concentrations were reduced from 2.93 mm/m3 and 0.31 mm/m3 in 1990 to 0.39 mm/m3 and 0.06 mm/m3 in 1996, respectively. The respective annual average concentrations of lead in both the roadside and in the general areas of 0.1 mm/m3 and 0.06 mm/m3 in 1996 in which leaded gasoline has been banned in Thailand are well below the WHO recommended guideline for annual average concentration which is in the range of 0.5 to 1.0 mm/m3. In conclusion, lead in the ambient air is on longer a problem for Thailand.