Evidence is accumulating that intronic polymorphic CA-repeats may play a role in gene expression. In this work, we investigated if a polymorphic CA short tandem repeat (STR) located at the first intron of the IGF1 pig gene influences plasma IGF1 concentration in pigs as well as phenotypic variation at growth and fatness traits. We measured plasma IGF1 levels at 1-4 time points from 35 until 215 days of age in 340 performance-tested Landrace and Duroc pigs previously genotyped for the IGF1 STR. Data were analyzed within breed using a linear mixed model with a covariate on the number of CA repeats. At least five alleles were segregating in each breed, differing in 1-7 repeats. The results showed that, in each breed, circulating IGF1 at 160 days of age increased with the length of the shortest allele, accounting for an average trend of 4.38±1.28 ng/mL of IGF1 per additional repeat (P=0.001). Longer repeats were associated with early growth in Landrace boars (1.92 ±0.92 kg at 160 days per CA, P=0.038) and with backfat thickness (-0.57±0.20 mm per CA, P=0.005) and lean content (7.52±3.00 g/kg per CA at 105 kg, P=0.013) adjusted for carcass weight in Duroc barrows, as expected from the effect of circulating IGF1 on these traits. The consistency of the results across populations supports the hypothesis that the length of the CA-repeats at intron 1 of the IGF1 gene is associated to circulating IGF1 levels, and that this effect is not neutral with respect to growth and fatness.