Three diamond workers had occupational asthma attributed to the inhalation of cobalt powder. The exposure originated from high speed polishing disks with an abrasive consisting of microdiamonds cemented in extra fine cobalt not alloyed to tungsten carbide. The bronchoconstriction progressed towards the end of working-days; it was especially pronounced in the absence of an adequate exhaust ventilation; and it could be accompanied by rhinitis and chest tightness. Cobalt inhalation challenge tests were positive in all three patients, and exposure to cobalt temporarily increased nonspecific hyperreactivity.