An n-by-n matrix is called a Iir-matrix if it is one of (weakly) sign-symme
tric, positive, nonnegative P-matrix, (weakly) sign-symmetric, positive, no
nnegative P-0,P-1-matrix, or Fischer, or Koteljanskii matrix. In this paper
, we are interested in Pi-matrix completion problems, that is, when a parti
al Pi-matrix has a Pi-matrix completion. Here, we prove that a combinatoria
lly symmetric partial positive P-matrix has a positive P-matrix completion
if the graph of its specified entries is an Pi-cycle. In general, a combina
torially symmetric partial Pi-matrix has a Pi-matrix completion if the grap
h of its specified entries is a 1-chordal graph. This condition is also nec
essary for (weakly) sign-symmetric P0- or P-0,P-1-matrices. (C) 2000 Elsevi
er Science Inc. Ail rights reserved. AMS classification: 15A48.