The first documented mumps outbreak in the workplace took place between August and December of 1987, when 119 cases of mumps occurred among employees at three Chicago futures exchanges and their household contacts. Twenty-one patients developed 23 complications, and nine persons were hospitalized. Total direct and indirect economic costs associated with the outbreak were $120,738; the cost per case was $1473. Only three patients had written documentation of mumps immunization. The outbreak is consistent with the recent changing epidemiology of mumps and the increase in reported cases in the adolescent and young adult populations. Mumps vaccine was licensed in 1967, but its distribution was limited until 1977 when the Immunization Practices Advisory Committee of the US Public Health Service recommended it for universal use. As a result, a cohort of adolescents and young adults under-immunized against mumps and underexposed to disease is now entering the work force. Vaccination of susceptible employees could prevent the substantial health impact of mumps.