I found these article on the net

HMB

HMB—known to scientists as beta-hydroxy beta-methylbutyrate—is a metabolite of the essential amino acid leucine. HMB became popular as a muscle and strength building supplement in the 1990s due to speculation that it might limit muscle protein degradation following resistance exercise and enhance positive nitrogen balance. The effects of HMB on muscle and strength gains resulting from resistance training have since been investigated in numerous studies. Some have shown that it is effective, while others have shown that it is not.

The majority of studies showing benefits have been poorly designed. A majority of the well-designed studies have shown no benefit. Also, most studies showing beneficial effects of HMB supplementation have involved beginning weightlifters, while most studies involving trained weightlifters have shown no benefit. Looking at the total pool of research on HMB I can say with confidence that HMB supplementation is not worth the $50-60 per month it would cost you.