We consider the fourth-order boundary value problem u'''' = f(t,u,u"), 0 < t < 1 u(0) = u(1) = u"(0) = u"(1) = 0 where f(t,u,p) = au - b p + o(vertical bar(u,p)vertical bar) near (0, 0), and f(t,u,p) = cu - dp + o(vertical bar(u,p)vertical bar) near infinity. We give conditions on the constants a, b, c, d that guarantee the existence of positive solutions. The proof of our main result is based upon global bifurcation techniques. (c) 2004 Elsevier Inc. All rights reserved.