Gray leaf spot (Pyricularia grisea [Cooke] Sacc.) is a major disease of perennial ryegrass (Lolium perenne L.) turf in the United States.    Conidia are an important source of primary and secondary inoculum for gray leaf spot; however, their seasonal airborne distribution patterns are unknown. Estimated concentrations of airborne P. grisea conidia were determined in the field between May and October from 2000 to 2004 in Maryland.  Conidia were monitored using a 7-day volumetric spore sampler in 0.6 m³ of air hr^-1 through a 14 by 2 mm orifice located 45 cm above the ground.  Total conidia collected each day and cumulative seasonal concentrations were determined.  Conidia of P. grisea were observed within air samples as early as 4 June.  Greatest airborne concentrations of P. grisea conidia generally were detected between mid-August and September.  During peak disease activity in adjacent stands of perennial ryegrass, as many as 5500 conidia were collected in a single day.  Disease severity in perennial ryegrass was not correlated with the total number of conidia collected in each season.  In all years, conidia concentrations declined between late September and mid-October.  Relatively large numbers of conidia, however, were collected in July (2002) and October (2002 and 2004), and conidia were detected as late as 31 October.