The impact of music on human cognition has a distinguished history as a research topic in psychology. The focus of the present study was on investigating the effects of music instruction on the cognitive development of preschool children. From a sample of 154 preschool children of Tehran kindergartens, 60 children aged between 5 and 6 were randomly assigned to two groups, one receiving music lessons and the other (matched for sex, age and mother's educational level) not taking part in any music classes. Children were tested before the start of the course of music lessons and at its end with 4 subtests of the Tehran-Stanford-Binet Intelligence Scale (TSB). The experimental group participated in twelve 75-min weekly music lessons. Statistical analysis showed significant IQ increase in participants receiving music lessons, specifically on the TSB verbal reasoning and short-term memory subtests. The numerical and visual/abstract reasoning abilities did not differ for the two groups after lessons. These data support studies that found similar skills enhancements in preschool children, despite vast differences in the setting in which the instruction occurred. These findings appear to be consistent with some neuroimaging and neurological observations which are discussed in the paper.