Employers are beginning to realize that they face a nearly invisible but significant drain on productivity: presenteeism, the problem of workers' being on the job but, because of illness or other medical conditions, not fully functioning. By some estimates, the phenomenon costs U.S. companies over 150 billion dollars a year--much more than absenteeism does. Yet it's harder to identify. You know when someone doesn't show up for work, but you often can't tell when, or how much, poor health hurts on-the-job performance. Many of the health problems that result in presenteeism are relatively benign. Research in this emerging area of study focuses on such chronic or episodic ailments as seasonal allergies, asthma, headaches, depression, back pain, arthritis, and gastrointestinal disorders. The fact is, when people don't feel good, they simply don't perform at their best. Employees who suffer from depression may be fatigued and irritable--and, therefore, less able to work effectively with others. Those with migraine headaches who experience blurred vision and sensitivity to light, not to mention acute pain, probably have a hard time staring at a computer screen all day. A number of companies are making a serious effort to determine the prevalence of illnesses and other medical conditions that undermine job performance, calculate the related drop in productivity, and find cost-effective ways to combat that loss. Indeed, researchers have discovered that presenteeism-related declines in productivity sometimes can be more than offset by relatively small investments in screening, treatment, and education. So organizations may find that it pays to make targeted investments in employees' health care--by covering the cost of allergy medication, for instance, or therapy for depression.