One hundred fifteen subjects took part in a double-blind study comparing the efficacy and safety of 0.5% malathion lotion (Prioderm lotion) and the alcoholic vehicle of the lotion as pediculicides in the treatment of head lice. The subjects, with a mean age of 9 years, had long-standing lice infestations and at least 20 viable ova. The subjects were treated with the test preparation, which was allowed to remain on the hair for 12 hours. Ovicidal and pediculicidal efficacy for the malathion lotion was significant; overall clinical evaluation by the investigators showed a significant number of moderate to marked therapeutic responses to malathion lotion. Only one adverse reaction (in the malathion lotion group) was reported, the stinging of pyodermal lesions in reaction to the alcoholic vehicle. Other concomitant scalp diseases were unchanged or improved at the end of the study.