One of the ways that snarky commentators will try to deflate a growth story is by referencing the trite-and-true cliché that "trees don't grow to the sky." That's true, but there's a big difference between a bonsai and a sequoia. Sure, Motley Fool Stock Advisor pick UnitedHealth's (NYSE:UNH) growth will have to slow someday, but that day doesn't seem to have come just yet.

United Health posted another ho-hum great second quarter. Revenue grew 28% to more than $11 billion, operating margins improved by almost a full percentage point, and net income climbed 36%.

Not surprisingly, cash flow continues to be strong as well. Operating cash flow is up 30% for the first six months of the year, and free cash flow is up by a similar 29% to more than $2 billion. With such ample cash flow United Health bought another 19 million shares during the quarter, though shares outstanding for the period still rose by 3.5%.

By and large, United Health continues to succeed by improving upon its already formidably good past performance. The medical care ratio, which measures medical costs as a percentage of premiums and fees, declined again. The claims cycle, which reflects the length of a claim from event to payment, improved. The company also continues to add new members. In other words, more of the same but better.

As I discussed just last week, the acquisition of PacifiCare (NYSE:PHS) should allow United Health to grow its business in the West in general and among seniors in particular. As you might expect, there really hasn't been much in the way of new information about this deal since the initial announcement, although management still remains confident that the deal will close before year end.

Even though a lot of analysts are sounding the alarm about HMOs and health insurers, I'm beginning to wonder just how bad things would actually be for United Health if and when the insurance business turns down.

Given its size, the company has considerable leverage when it comes to dealing with health-care providers, health-care companies, and clients. That's not to say that other large players such as Aetna (NYSE:AET) or Wellpoint (NYSE:WLP) couldn't mess things up with aggressive pricing (they're both big too, you know), but I don't see that as especially likely.

What's more, even as external growth may slow a bit and/or health-care costs may rise, United Health is still running an incredibly cash-rich business. Like many companies, then, it may have the option to harvest more cash out of the business if or when growth slows -- using that cash to pay dividends and/or buyback shares and keep the apparent EPS growth rate moving forward.

Can HMOs make your portfolio healthy?

You can read Tom Gardner's analysis of United Health in Motley Fool Stock Advisor. Click here to learn more.

Fool contributor Stephen Simpson has no financial interest in any stocks mentioned (that means he's neither long nor short the shares).