In this paper we investigate the problem of finding a 2-connected spanning subgraph of minimal cost in a complete and weighted graph G. This problem is known to be APX-hard, for both the edge and the vertex connectivity case. Here we prove that theAPX-hardness still holds even if one restricts the edge costs to an interval [1, 1+ε], for an arbitrary small ε > 0. This result implies the first explicit lower bound on the approximability of the general version (i.e., for arbitrary graphs) of the problem. On the other hand, if the input graph satisfies the sharpened -triangle inequality, then a ( 2 3 + 3 · 1− ) -approximation algorithm is designed. This ratio tends to 1 with tending to 1 2 , This work was partially supported by DFG-grant Hr 14/5-1, the Research Project GRID.IT, partially funded by the Italian Ministry of Education, University and Research, the European projects RTN ARACNE (Contract No. HPRN-CT-1999-00112) and IST FET CRESCCO (Contract No. IST-2001-33135). A preliminary version of this paper appeared in Proc. of the 22nd Foundations of Software Technology and Theoretical Computer Science (FSTTCS’02), Vol. 2556 of Lecture Notes in Computer Science, Springer-Verlag, 2002, 59–70. ∗ Corresponding author. Dipartimento di Informatica, Università di L’Aquila,ViaVetoio, 67010 L’Aquila, Italy. E-mail addresses: email@example.com (H.-J. Böckenhauer), firstname.lastname@example.org (D. Bongartz), email@example.com (J. Hromkovič), firstname.lastname@example.org (R. Klasing), email@example.com (G. Proietti), firstname.lastname@example.org (S. Seibert), email@example.com (W. Unger). 0304-3975/$ see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tcs.2004.06.019 138 H.-J. Böckenhauer et al. / Theoretical Computer Science 326 (2004) 137–153 and it improves the previous known bound of 3 2 , holding for graphs satisfying the triangle inequality, as soon as < 7 . Furthermore, a generalized problem of increasing to 2 the edge-connectivity of any spanning subgraph of G by means of a set of edges of minimum cost is considered. This problem is known to admit a 2-approximation algorithm. Here we show that whenever the input graph satisfies the sharpened -triangle inequality with < 3 , then this ratio can be improved to 1− . © 2004 Elsevier B.V. All rights reserved.