Mutations in the thymidine kinase and DNA polymerase genes of herpes simplex virus (HSV) might confer resistance to antiviral drugs, particularly in immunocompromised patients who suffer from chronic and/or disseminated lesions. The patterns of cross-resistance and neurovirulence in mice of several DNA polymerase mutants selected under pressure of foscarnet (PFA) and different acyclic nucleoside phosphonates (ANPs), including (S)-3-hydroxy-2-phosphonylmethoxypropyl (HPMP) derivatives of adenine (HPMPA) and cytosine (HPMPC, cidofovir) and 2-phosphonylmethoxyethyl (PME) derivatives of adenine (PMEA) and 2,6-diaminopurine (PMEDAP), were investigated. The mutants were derived from the HSV-1 strain KOS following either single or multiple steps of selection with PFA (V714M, A719V, 5724N and T821M), PMEA (S724N, L802F and R959H), PMEDAP (Q618H, S724N, S724N+D1070N), HPMPC (V573M, R700M and K960R) or HPMPA (W998L, L1007M and 11028T). These amino acid substitutions were located in different subdomains of the HSV-1 DNA polymerase, either in conserved or non-conserved regions. The sensitivity of the mutants to a new class of ANPs, the 6-(2-[phosphonomethoxy]alkoxy)pyrimidines HPMPO-DAPy and PMEO-DAPy, was investigated. Cross-resistance between the HPMP derivatives and HPMPO-DAPy, on the one hand, and between the PME derivatives and PMEO-DAPy, on the other hand, was observed. Different degrees of cross-resistance between PME derivatives, PMEO-DAPy, PFA and acyclovir were noticed. The mutants ranged from exhibiting near wild-type neurovirulence (V714M, A719V, 5724N and L1007M) to significant attenuation (Q618H, S724N+D1070N, L802F, R700M, K960R, W998L and 11028T) or higher levels of attenuation (V573M). It appears that drug-resistant mutants arising under the pressure of HPMP derivatives have the lowest levels of neurovirulence.