AbstractTwenty‐seven welders and a control group of 27 workers not engaged in welding were interviewed at nine factories to assess the incidence of metal‐fume fever over a six‐month period.Twenty‐two welders had one or more febrile episodes over the six‐month period, compared with 12 in the control group. There were 32 episodes in the welding group, and 22 in the controls. However, much of the difference between the groups was in the number of episodes of more than three days duration, that is, longer than the duration of typical metal‐fume fever.Analysis of the figures relating to short‐term episodes (three days or less in duration), suggested that the joint effects of smoking and welding increase a worker's susceptibility to short‐term fever. This apparently synergistic effect, when viewed against similar findings for smoking and asbestos dust exposure and for smoking and radioactive mine‐dust exposure, underscores the oft‐overlooked interplay of aetiologic factors in producing unexpectedly large chan