Paneth cell count at onset correlates with GI GVHD response to treatment. Mean Paneth cell counts per hpf (horizontal lines show median of the means) were significantly different (P < .0001) in biopsies obtained from control patients without GI GVHD (n = 26) and at the onset of GI GVHD in patients whose 4-week response to treatment was complete response (CR, n = 56), partial response (PR, n = 24), or no response/progression (NR, n = 34). The interquartile range of mean Paneth cell counts per hpf was 5.7 to 16.9 for CR, 1 to 12.5 for PR, and 0 to 6.1 for NR.