Mercer said employers were motivated to open their own clinics by two primary factors: the desire to more closely manage employee health, and concerns over access to health care for their employees.

"Of the 134 respondents [that employed more than 5,000 workers], 72 percent of those whose clinics provide general medical services said that managing employee health risk and chronic conditions is an important objective for the clinic," Mercer said.  "For more than two-thirds of survey respondents (68 percent), improving access to care was also an important objective."

Employers indicated they were satisfied with their clinics — 85 percent said they were successful, although only 41 percent were measuring clinic operations on  a return-on-investment basis. Nearly half (49 percent) said they offer employees the option to select the onsite clinic as their primary care provider.

A possible damper on clinic expansion: recent Internal Revenue Service guidance that suggests the cost of an onsite clinic may contribute to Cadillac tax qualification. But the survey showed that these large employers generally aren't convinced they'll be hurt by the tax, particularly if the clinics offset penalties.

Among the findings for clinic performance:

• 63 percent say their clinics have successfully reduced lost work days;

• 58 percent say clinics have been successful in helping members control chronic conditions;

• Of those that developed an ROI for clinics, 23 percent reported a range of 1-1.99, 13 percent reported an ROI of 2 or higher, and 5 percent reported ROI of less than 1.

Asked about clinic services, the breakdown was:

• Biometric screenings: 77 percent

• Urgent care: 73 percent

• Preventative care: 71 percent

• Chronic disease management: 63 percent

• Chronic condition coaching: 60 percent

• Lifestyle management programs: 59 percent

• Weight management: 56 percent

• Pharmacy: 38 percent

• Mental health/EAP: 26 percent

Another sign that onsite clinics are performing well and are here to stay: More employers are designing them as "medical homes."

"The patient-centered "medical home" is a delivery model through which patients — often those who are high-risk or chronically ill — can have their care coordinated by a primary care physician, nurse practitioner, or physician assistant," Mercer reported. "Using the worksite clinic as a medical home allows employers to offer a greater level of care to the higher-risk, medically challenging employees and dependents who typically account for the largest portion of health care spending. Three years ago, 13 percent of survey respondents with general medical clinics used their clinic as a medical home. That's doubled, to 26 percent of the respondents to this year's survey, with another 11 percent considering it."