Blue Cross & Blue Shield of Utah may not have financial reserves as large as many of its sister health insurers across the nation, but 1990 was the company's most successful in its 48-year history in terms of earnings and membership growth.

The Utah insurance plan Friday reported net earnings of $10.01 million on revenue of $304.20 million for the year while paying out $265.92 million in health insurance claims.In 1989, the company reported net earnings of $9.14 million on revenue of $249.31 million. In 1988, earnings were $797,000.

Also during the past year, 28,097 new members boosted total enrollment in the Utah plan to 422,030 subscribers, the highest ever. The plan also administers Medicare claims for some 150,000 Utahns.

Jed H. Pitcher, president and chief executive officer, said the 1990 gains will be added to the company's reserves "to continually ensure our ability to meet the claims needs of our subscribers in the future."

He added that $400,000 of the earnings have been set aside for the Utah Life and Disability Insurance Guarantee Association, a guarantee fund for the state's insurance companies. Seven insurance companies doing business in Utah have gone bankrupt in recent years, the most visible being Maxicare and American Protectors Inc.

As Utah's largest insurer, Blue Cross & Blue Shield is the largest contributor to the guarantee fund.

Pitcher credited the financial turnaround of the past two years to control of administrative expenses, internally managed care programs and close monitoring of financial data.