To evaluate the frequency of Gilles de la Tourette's syndrome (TS) in children, 3034 students in three schools in a single school district in greater Los Angeles were monitored frequently over a 2-year period by a school psychologist thoroughly familiar with the symptoms of the disorder. A portion of the cases were also evaluated in a TS clinic. A total of 14 males fulfilled the Tourette Syndrome Association research criteria for definite TS. When corrected for the number of students in special education classes in the monitored schools, the frequency of definite TS in males was 1 in 152. An additional 7 males who differed only in that they were not observed for a full year were termed definite TS less than 1 year. When the two groups were combined, the frequency of definite TS was 1 in 95 for males and 1 in 759 for females. These figures do not include an additional 10 males diagnosed as having definite transient tic disorder, 2 males diagnosed as having probable TS, and 10 males diagnosed as having possible TS. In addition to tics, most of these children had problems with attention span, obsessive compulsive behavior, and learning and/or conduct disorders. Seventy percent of the students with definite TS or definite TS less than 1 year were in special education classes. Twelve percent of the children in special education classes had definite TS or definite TS less than 1 year, and 28% were in one of the diagnostic categories of definite, probable, or possible. All of the 10 definite TS patients that were seen in the clinic had attention-deficit hyperactivity disorder.(ABSTRACT TRUNCATED AT 250 WORDS)