|Abstract||OBJECTIVE: To evaluate the efficacy and safety of 4% dimeticone lotion for treatment of head louse infestation. DESIGN: Randomised controlled equivalence trial. SETTING: Community, with home visits. PARTICIPANTS: 214 young people aged 4 to 18 years and 39 adults with active head louse infestation. INTERVENTIONS: Two applications seven days apart of either 4.0% dimeticone lotion, applied for eight hours or overnight, or 0.5% phenothrin liquid, applied for 12 hours or overnight. OUTCOME MEASURES: Cure of infestation (no evidence of head lice after second treatment) or reinfestation after cure. RESULTS: Cure or reinfestation after cure occurred in 89 of 127 (70%) participants treated with dimeticone and 94 of 125 (75%) treated with phenothrin (difference -5%, 95% confidence interval -16% to 6%). Per protocol analysis showed that 84 of 121 (69%) participants were cured with dimeticone and 90 of 116 (78%) were cured with phenothrin. Irritant reactions occurred significantly less with dimeticone (3/127, 2%) than with phenothrin (11/125, 9%; difference -6%, -12% to -1%). Per protocol this was 3 of 121 (3%) participants treated with dimeticone and 10 of 116 (9%) treated with phenothrin (difference -6%, -12% to -0.3%). CONCLUSION: Dimeticone lotion cures head louse infestation. Dimeticone seems less irritant than existing treatments and has a physical action on lice that should not be affected by resistance to neurotoxic insecticides.