This paper examined the effects of training in creative problem-solving on intelligence. We revisited Stankov's report on the outcomes of an experiment carried out by R. Kvashchev in former Yugoslavia that reported an IQ increase of seven points, on average, across 28 tests of intelligence. We argue that previous analyses were based on a conservative analytic approach and failed to take into account the reductions in the IQ test variances at the end of the three-years' training. When standard deviations of the initial test and 2nd retest were pooled in the calculation of the effect sizes, the experimental group's performance was 10 IQ points higher on average than that of the control group. Further, with the properly defined measures of fluid and crystallized intelligence, the experimental group showed a 15 IQ points higher increase than the control group. We concluded that prolonged intensive training in creative problem-solving can lead to substantial and positive effects on intelligence during late adolescence (ages 18-19).