The region off the east coast of Sakhalin is thought of as an important pathway of dense shelf water (DSW) from its production region in the northwestern Okhotsk Sea to the southern Okhotsk Sea. From July 1998 to June 2000, the first long-term mooring experiment was carried out in this region to observe the southward flowing East Sakhalin Current (ESC) and DSW. Moored and associated hydrographic data show considerable modification of cold dense water via mixing with warm offshore water in the slope region off northern Sakhalin. Significant onshore eddy heat flux was observed at the northernmost mooring (54.9¿N), which suggests the occurrence of baroclinic instability. The eddy heat flux was not significant farther south. At moorings along 53¿N, cold anticyclonic eddies were identified that were consistent with isolated eddies seen in the hydrographic data. The three years of hydrographic data also showed large differences in extent and properties of DSW. Furthermore, the mooring data show that seasonal variability of DSW was quite different in the two years. The average DSW transport for σ$theta$ > 26.7 evaluated using the moored data at 53¿N for 1 year (1998--1999) was ~0.21 Sv (= 106 m3 s-1). This value is at the lower end of the previous indirect estimates. Along with the DSW modification, this transport estimate indicates that DSW was not only carried southward by the ESC but was spread offshore by eddies off northern Sakhalin.