The ability of azathioprine to reduce the maintenance prednisolone requirement of 31 patients with polymyalgia rheumatica (PMR) or giant cell arteritis (GCA), or both, was tested in a double-blind placebo controlled study over one year. Clinical and laboratory assessments were made at four-weekly intervals over a period of 52 weeks. A statistically significant difference (p less than 0.05) in mean prednisolone dose was noted between the two groups at the end of 52 weeks, there being a fall in steroid requirement in the azathioprine treated group.