OBJECTIVE Cystatin C, a low molecular weight protein, is produced by nucleated cells, filtered by glomeruli, and degraded by tubules at a constant rate. Its serum concentration has been proposed as a marker of GFR. Its size should make it dialyzable. It is hypothesized that serum cystatin C levels are influenced by the method and intensity of dialysis received. METHODS This is a cross-sectional pilot study of cystatin C in functionally anephric dialysis patients. It was measured predialysis in 14 patients on conventional (3 to 5 h, 3 x wk) hemodialysis; eight on nocturnal hemodialysis (three to seven nights, 6 to 8 h); three on daily hemodialysis (6 d, 1(1/2) to 2(1/2) h); and 10 on automated peritoneal dialysis. All had urea kinetic studies and values for single pool Kt/V (Sp Kt/V), standard weekly Kt/V (Std Kt/V), and protein equivalent of nitrogen appearance (nPNA; g/kg/d). C reactive protein (CRP; mg/L) and thyroid stimulating hormone (TSH; mIU/L) were measured as factors known to influence cystatin C. RESULTS There was no correlation between cystatin C and Sp Kt/V, but there was a significant inverse linear correlation with Std Kt/V and there were significant differences between treatment modalities in cystatin C levels and in Std Kt/V. The estimation of cystatin C was reliable and stable over 3 to 6 wk and its levels uninfluenced by nPNA, CRP, or TSH. CONCLUSIONS Serum cystatin C levels are influenced by the method and intensity of dialysis and may have a role in treatment adequacy monitoring.