This study was conducted to estimate the prevalence of head lice in primary school children in Chiangyuen district and to compare the efficacy of a 3 hour versus a 12 hour application of 1% gamma benzene hexachloride (1% lindane). Two schools were selected at random. Children who were infested with head lice received a 12 hour application of 1% lindane in the first school, while those in the second group received a 3 hour application. While none of the boys examined were infested, there were 39 (41.1%) and 41 (44.1%) female children with pediculosis capitis in two schools, respectively. Thirty-seven out of 38 and 23 out of 41 infections were cured when the drug was applied for 12 and 3 hours, respectively. This study showed that there is still a high prevalence of pediculosis in female school children and that 3 hour applications of lindane are less effective (P < 0.05) in curing head lice infestations than 12 hour applications. . . .