The history and development of pyrethrum, the natural pyrethrins and synthetic pyrethroids and their insecticidal properties, chemical structure and toxic and allergic side-effects are reported. Permethrin is stressed as a photostable insecticide that is very effective against a large variety of insects and mites with low mammalian toxicity and virtually no allergic side-effects. Only 10-20 min after application, permethrin (1% cream rinse or 0.5% in ethanol) proved to be safe, reliable and cosmetically acceptable in the treatment of infestations with head lice and the prevention of reinfestations, and also in failures with lindane owing to the development of tolerance in the lice. The same was true of 5% permethrin cream (2.5% in children below 5 years of age) used in the treatment of scabies. Permethrin is absorbed percutaneously in only small amounts, is metabolized rapidly in the skin and excreted in the urine. A single "head to toe" application is ideal for eradication programmes allowing lice to be targetted and the prevalence of secondary bacterial infections decreased at the same time. Benzyl benzoate has an irritant potential, and lindane has been reported to exert CNS toxicity in a few anecdotal cases, in particular in small children or after repeated applications in patients with scabies crustosa, and permethrin was distinctly superior to both of these. This is documented by the results obtained in the treatment of 48 children and 56 adults suffering from scabies. Permethrin is recommended in scabies therapy in premature infants, small children, patients with seizures and neurological complications, in treatment failures with lindane entailing the need to repeat the therapy, in scabies crustosa and in pregnant women and nursing mothers.