This study was conducted to investigate differences in spatial visualization abilities and effects of instruction on spatial visualization skills of fifth through eighth grade students by grade, sex, and site. About 1,000 students from three sites, representing a wide range of socioeconomic status, participated in the study. The spatial visualization unit engaged students in concrete activities, building and drawing solids made of cubes. The instrument used was the MGMP Spatial Visualization Test, with a test-retest reliability of .79, and Cronbach’s reliability coefficients for various groups of students ranged from .72 to .86 on the pretest and from .82 to .88 on the posttest. Before instruction, there were significant differences in spatial visualization performance by grade (increasing with age), by sex (favoring boys), and by site (increasing with socioeconomic status). After instruction, fifth through eighth grade students profited considerably from instruction, and the gain was similar for boys and girls despite initial sex differences. Retention of effects persisted after a 4-week period and after 1 year.