Counts of birds attending leks traditionally have been used as an index to the population size of greater sage-grouse (Centrocercus urophasianus) and, more recently, as a means to estimate population size. The relationship between this index and the actual population has not been studied. We used intensive counts of individually marked and unmarked greater sage-grouse on leks to evaluate how sex and age of birds, time of day, and time of season impact lek-attendance patterns and lek counts. These within-season sources of variation need to be considered when estimating detection probability of birds on leks and ultimately adjusting the lek-count index to estimate true population parameters. On average, 42% of marked adult males, 4% of marked hens, and 19% of yearling males were observed on leks per sighting occasion with all 15 known leks being intensively counted. We discovered that lek counts as currently conducted may be useful as an index to greater sage-grouse populations, but standardization of protocols is needed to allow for better spatial and temporal comparisons of lek-count data. Also the probability of detecting birds on leks must be estimated in order to relate lek counts to population parameters. Lastly, we evaluated use of the bounded-count methodology for correcting lek-count data. We showed large biases associated with this technique and below-nominal coverage of confidence intervals even at large numbers of counts, demonstrating the unreliability of the bounded-count method to correct lek-count data.