Virtually all employes in the nation's medium and large firms in 1984 had health insurance and life insurance coverage, and about four-fifths were covered by private pension plans, according to a survey released yesterday by the Labor Department.

The survey covered firms with a minimum of 100 to 250 employes, depending on the industry, and provided data on the employe benefits enjoyed by about 21 million full-time workers. Employes in these larger firms generally have better benefits than those in smaller firms. For example, while four-fifths of workers in the medium and large firms had pension coverage, according to the survey, for the economy as a whole, only slightly more than half of all workers are covered by pensions.

The Labor Department's new survey of medium and large firms showed these patterns in benefit coverage:

*97 percent of the full-time employes surveyed had health insurance covering them for most categories of hospital and medical costs. Employers paid the full cost of the insurance for three-fifths of the workers and two-fifths of their dependents, but the remaining two-fifths of workers contributed to the cost. About three-fifths of the workers were in plans that extended coverage to retirees in some form. Although nearly all workers had basic health and hospital coverage, vision care was less common (30 percent), and only 14 percent had coverage for hearing care and 8 percent for routine physicals. More than two-fifths were covered for home health care and one in 10 for hospice care if terminally ill.

*96 percent of the workers were covered by a life insurance policy roughly equal to annual earnings, usually wholly employer-financed.

*82 percent of the workers were covered by private pension plans, with the employer paying the full cost for all but a handful. Most of the plans based benefits on earnings during the last five years of employment. Common eligibility requirements: age 65 without a specified length of service, age 62 with at least 10 years of service, or at any age after 30 years of service. In most cases, early retirement was allowed with a reduced benefit. For the average worker, the combined pension and basic Social Security benefit after 30 years of service was 74 percent of final salary if the latter was $15,000; 54 percent of final salary if the latter was $30,000, and 48 percent of final salary if the latter was $40,000.

*Paid holidays averaged 9.8 days per year.

*Vacations averaged 8.6 days after one year, 15.6 days after 10 years, 18.2 days after 15 years and 22.3 days after 25 years.

*94 percent had some coverage against loss of pay because of short-term illness: sick leave or accident and sickness insurance or both.

*Most had some protection against long-term disability: 47 percent had long-term disability insurance and 46 percent had pension plan provisions allowing them to retire immediately if disabled.

*Nearly nine-tenths of all workers were eligible for paid funeral leave, and about a fifth for profit-sharing plans.