Many private conifer plantations are degraded by poor management in Japan. Their rehabilitation requires assessing their actual forest conditions, which few studies have addressed. This study thus analyzed municipal administrative survey data on the poorly managed private conifer plantations. The results showed that these forest stands were generally so dense that the mean height-to-diameter ratio (HDR) exceeded 80 across all the stands, whereas they were so diverse that their relative spacing index (Sr) ranged from 10 to 20. Canopy height and the mean trunk diameter and HDR of a stand were smaller in stands with greater Sr, indicating the close relationships between Sr and stand properties. The frequency distribution of HDR in each stand was positively skewed, indicating that most trees were dominant or codominant with lower HDR values. With increasing Sr, the distribution retained its shape but shifted in location toward lower HDR values. Simulations of stand structure after low thinning highlighted the necessity of more intense thinning in stands with smaller Sr in lowering their stand HDR sufficiently, but revealed that in particularly dense stands (Sr ≈ 10), a single thinning operation was insufficient to achieve a post-thinning stand HDR of ≦80 even with a severe thinning removing 50％ of the trees. Within-stand variation in HDR, however, was so large that on average, nearly 20％ of the trees had HDR≦80 and qualified as crop trees even in the dense stands where Sr ≈ 10, highlighting the applicability of crop tree release to these overcrowded stands.