In 325 cases of industrial chemical cyanosis notified to Her Majesty's Factory Inspectorate for 1961-80 the incidence of poisonings showed considerable seasonal variation with substantially greater numbers occurring in the summer months. A correlation between the number of poisonings in any one year and the hotness of that summer was also shown. The vast majority of incidents occurred during the manufacture of chemicals or dyestuffs, and two particular workplaces were responsible for 70% of the cases. Poisoning by amino compounds appear to produce early cyanosis whereas poisoning by nitro compounds tended to produce delayed cyanosis. These latter compounds were also more likely to produce anaemia. Methaemoglobin was determined in 45% of cases with results ranging from less than a few per cent to over 50%. Despite concentrations of MetHb over 50%, several workers complained only of headache and their blue appearance.