In an occurrence very similar to that of competitor UnitedHealth Group (NYSE:UNH), blue-chip health insurer WellPoint (NYSE:WLP) took one on the chin yesterday despite reporting impressive Q1 earnings. Its share price closed down 3.2% compared with the previous day's close as investors shifted their focus away from rising profits and toward a climbing benefit-expense ratio.

On a per-share basis, the company recorded a 15.4% increase in net income as operating revenue rose 7.5% compared with the year-ago quarter. Company management attributed the improvement in both metrics to premium-rate increases and membership growth. Other positive trends during the quarter for WellPoint shareholders were the company's repurchase of $1.3 billion of common stock and an upward revision of full-year results to $5.55 per share.

Nonetheless, shareholders of WellPoint felt the heat of a 60-basis-point increase in the organization's benefit expense ratio, which rose to 81.8%, versus 81.2% in the prior-year quarter. This trend itself was the driving force behind the dip in the company's stock price. The rising benefit-expense ratio stems from rising claims in WellPoint's state-sponsored programs, as well as the growth of its Medicare Part D business, which has traditionally seen a notably higher benefit-expense ratio in the early stages of the year.

Shares of WellPoint have remained relatively flat year-to-date, and it now appears that the market is waiting to see whether these mega-cap insurers will be able to continue to maintain their history of strong earnings growth in the wake of rising benefit expense ratios. It will be interesting to see whether Aetna's (NYSE:AET) shares experience a better fate today than did those of WellPoint. Before the opening bell, Aetna reported slight improvements in both its total medical benefit ratio and its commercial medical benefit ratio. All of these insurers are seeing their earnings flourish, but investors might want to remain a bit more cautious, as the market is clearly escalating its scrutiny of benefit expense ratios.

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Fool contributor Billy Fisher has no financial interest in any stocks mentioned. The Fool has a disclosure policy.