266 eyes were examined by the slit lamp 24 hours after uncomplicated intracapsular cataract extraction. An unexpectedly high incidence of 15% macroscopic hypopyons was found with a further 13% showing a trace. Possible aetiological irritative factors were investigated by individual exclusion, but did not materially alter the incidence. All the hypopyons spontaneously and rapidly regressed without specific treatment. The suggested explanation of hypopyon production is that operative plasmoid aqueous production, inflammatory reaction, and hyphaema, coupled with early mobilisation, allow gravitational sedimenting out of blood constituents, so that white cells will fall late, forming a layer above other blood constituents.