Quantifying differences in markup fluctuations between relatively competitive and non-competitive industries has received increasing attention in industrial organization and macroeconomics. We examine markup fluctuations in response to energy price and monetary changes, and the role played by product market competition. This paper contains several features that distinguish it from previous work. First, we separate the source of the shock, cost versus demand. Previous studies have typically used the unemployment or the capacity utilization rate to study industry markup fluctuations. However, these measures are likely to provide only an overall picture of fluctuations over business cycles, and may conceal important information about the underlying impacts of demand and cost changes. Second, we address an important criticism raised by Rotemberg and Saloner (1986) that differences in markup fluctuations between the low and high concentration industries may be an artifact of differences in the degree of labor hoarding. Third, since we have a longer time-series (1958-91), we are better able to examine the causes of the narrowing of the markup gap between high and low concentration industries during the mid-1970's (Domowitz, Hubbard and Petersen, 1986) and the subsequent widening of the markup gap during the mid-1980's (Salinger, 1990). Our key findings are as follows: (1) While data show that labor-hoarding is an important phenomenon, we provide evidence that the degree of labor-hoarding does not systematically vary between the low and high concentration industries. This addresses the Rotemberg and Saloner criticism and implies that the cyclicality of markups in concentrated industries are not an artifact of differences in the degree of labor hoarding. (2) For concentrated industries, markups increase with monetary expansion and decrease when energy prices increase; the quantitative impact of energy prices is greater than monetary changes. Markups of the low concentration industries are generally unaffected by monetary changes and the energy price elasticities are statistically insignificant. Collectively, these findings indicate that (a) markups are procyclical and (b) the energy price increases (decreases) during the 1970's (1980's) may have been a key factor contributing to the narrowing (widening) of the markup gap between high and low concentration industries. (3) Markups are procyclical over the industry-specific cycle. (4) Greater import competition lowers markups only in the more concentrated industries.