Paresis strikes most frequently during life's most productive years. It is seen approximately three times as frequently in males as females, and over twice as frequently in whites as in Negroes. The usual incubation period is 10 to 24 years after infection. The simple dementing type of psychosis is more frequent than all other types combined. In the absence of previous treatment the spinal fluid complement fixation reaction is always positive and the spinal fluid cell count is almost always elevated. The blood serologic test may, rarely, be negative.