Context :

Aims : HESS J1858+020 is a weak \gamma - ray source that does not have any clear cataloged counterpart at any wavelengths .
Recently , the source G35.6-0.4 was re-identified as a SNR .
The HESS source lies towards the southern border of this remnant .
The purpose of this work is to investigate the interstellar medium around the mentioned sources in order to look for possible counterparts of the very-high energy emission .

Methods : Using the ^ { 13 } CO J=1–0 line from the Galactic Ring Survey and mid-IR data from GLIMPSE we analyze the environs of HESS J1858+020 and SNR G35.6-0.4 .

Results : The ^ { 13 } CO data show the presence of a molecular cloud towards the southern border of SNR G35.6-0.4 and at the same distance as the remnant .
This cloud is composed by two molecular clumps , one , over the SNR shell and the other located at the center of HESS J1858+020 .
We estimate a molecular mass and a density of \sim 5 \times 10 ^ { 3 } M _ { \odot } and \sim 500 cm ^ { -3 } , respectively for each clump .
Considering the gamma-ray flux observed towards HESS J1858+020 , we estimate that a molecular cloud with a density of at least 150 cm ^ { -3 } could explain the very-high energy emission hadronically .
Thus , we suggest that the \gamma - ray emission detected in HESS J1858+020 is due to hadronic mechanism .
Additionally , analyzing mid-IR emission , we find that the region is active in star formation , which could be considered as an alternative or complementary possibility to explain the very-high energy emission .

Conclusions :