Blame it on the economy. Blame it on the high monthly charges. Or blame it on the already high penetration.

Whatever be the reason, broadband growth in the U.S. slowed down considerably in the second quarter.

According to the Leichtman Research Group, broadband additions in the second quarter amounted to 51% of those in the second quarter of 2007, with cable reporting 85% as many additions as a year ago, and the telephone companies 23%.

Top cable companies are said to have added 670,000 subscribers in Q2, representing 76% of the net broadband additions for the quarter.

The slowdown in U.S. broadband growth does not bode well for upcoming broadband services like WiMAX from Clearwire.

The Leichtman study said the twenty largest cable and telephone providers in the US (representing 94% of the market) picked up 887,000 net additional high-speed Internet subscribers in the second quarter of 2008.

The top 20 broadband providers account for 65.1 million subscribers – cable companies account for 35.3 million broadband subscribers and telephone companies for 29.7 million.

The top cable broadband providers have a 54% share of the overall market, with a 5.6 million subscriber advantage over the top telcos.