A randomized double-blind, placebo-controlled, trial of 5,6 benzo-[alpha]-pyrone (coumarin, '56 BaP') and of diethylcarbamazine (DEC) is being performed on patients with filaritic lymphoedema and elephantiasis over two years, using matched groups. In 169 patients there were significant (1% level) reductions in the amounts of oedema for the patients taking 56 BaP. The excess limb volumes were reduced from 40 to 25% over two years. A similar, but less significant (5% level), improvement was found for the circumference measurements. The rate of reduction was increased when the initial amount of oedema was greater. This therapy, while much slower than many other methods of treatment, does convert a slowly worsening condition into a slowly improving one. There were no significant reductions in the amount of oedema following DEC treatment.