My Honda Civic 1999 is blowing out white smoke on startup ? A lot of smoke. What can it be ?

Tom is right, but if it is only on startup, it might mean your valve guides are bad- the oil drips down into the combustion chamber while the engine is off- that's an old engine, and it might be worn out rather than blown- I hope so, for your sake-

Are you a newbie with engines? What's the current mileage, how long has been smoking on start up, did you buy it used or new? Do you use it frequently/infrequently? Have you noticed your coolant "disappearing" from the reservoir? Does your dipstick tip looks milk whitish or like Mayo-ish? Do you keep up with the maintenance routine by religiously changing oil every 3-5K miles? or just turn on ignition, ride it and limited to filling the tank when low? Tom & JB are correct but unless you rolled the odometer already at least once there's no reason your head gasket should have failed and if they are correct it tells me you do not check your oil which in that case would have given you a heads up (if you are knowledgeable regarding how clean fresh engine oil should look). Be careful as a slick mechanic can hit you for over a grand to change a head gasket which you may not need!!!! See below: *Milky oil on the dipstick can indicate an engine problem. If noticing milky looking oil on the dipstick, then the first thing to check is the engine’s coolant. A leaky head gasket can allow coolant to pass into the oil system. When coolant and oil mix, or attempt to mix, the result is oil that looks milky. However, this can also be caused by a collection of moisture created by combustion. This type of moisture builds up because the engine was not allowed to reach, and maintain normal operating temperature. The latter can be fixed by properly warming the engine, while the former requires a new gasket. http://www.autos.com/car-maintenance/what-does-milky-oil-on-the-dipstick-mean