The study aimed to compare MspI polymorphisms in the 3rd intron of the porcine gene encoding the pituitary-1 transcription factor (Pit-1, renamed as POU1F1) among 5 breeds and to determine the associations between its genotypes and growth performance in a commercial pig population by using the PCR-RFLP technique. Significant differences in genotypic and allelic frequencies were found between the meat-type and fat-type breeds (P < 0.05), and between miniature pigs and others (P < 0.05). No breed deviated from the Hardy-Weinberg equilibrium (verified by chi-square test). The general linear model analysis revealed that higher body weight on day 180 (BW180) and average daily gain (ADG) were significantly associated with POU1F1 DD genotype (P < 0.05). The differences in BW180 and ADG between DD pigs and both CD and CC pigs were significant (P < 0.05), and the DD pigs had a significantly higher body weight on day 45 (BW45) and on day 70 (BW70) than CC pigs (P < 0.05). All measured growth traits, except for body weight at birth (BWB), showed higher values in DD pigs. The D allele had a favorable positive effect on growth traits. Thus POU1F1 is a potential major gene or marker for growth traits.