This is an old study, but an important one by William Kraemer:

Performance and muscle fiber adaptations to creatine supplementation and heavy resistance training. Med. Sci. Sport Exerc. 1999 vol 31 #8: 1147-1156.

Unlike other research studies this one involved experienced weight trainers in a progressive resistance training program over 12 weeks. The results were quite outstanding to say the least. However, it is important to note that Creatine was taken in the traditional fashion of Loading the first week, then going to a 5g maintenance dose. I suspect that the results would have been more significant if they had taken week 5 off, and re-loaded on week 6.

Medicine & Science in Sports & Exercise. 31(8):1147-1156, August 1999.
VOLEK, JEFF S.; DUNCAN, NOEL D.; MAZZETTI, SCOTT A.; STARON, ROBERT S.; PUTUKIAN, MARGOT; GOMEZ, ANA L.; PEARSON, DAVID R.; FINK, WILLIAM J.; KRAEMER, WILLIAM J.

Abstract:
Performance and muscle fiber adaptations to creatine supplementation and heavy resistance training. Med. Sci. Sports Exerc., Vol. 31, No. 8, pp. 1147-1156, 1999.

Purpose: The purpose of this study was to examine the effect of creatine supplementation in conjunction with resistance training on physiological adaptations including muscle fiber hypertrophy and muscle creatine accumulation.

Methods: Nineteen healthy resistance-trained men were matched and then randomly assigned in a double-blind fashion to either a creatine (N = 10) or placebo (N = 9) group. Periodized heavy resistance training was performed for 12 wk. Creatine or placebo capsules were consumed (25 g[middle dot]d-1) for 1 wk followed by a maintenance dose (5 g[middle dot]d-1) for the remainder of the training.

Results: After 12 wk, significant (P <= 0.05) increases in body mass and fat-free mass were greater in creatine (6.3% and 6.3%, respectively) than placebo (3.6% and 3.1%, respectively) subjects. After 12 wk, increases in bench press and squat were greater in creatine (24% and 32%, respectively) than placebo (16% and 24%, respectively) subjects. Compared with placebo subjects, creatine subjects demonstrated significantly greater increases in Type I (35% vs 11%), IIA (36% vs 15%), and IIAB (35% vs 6%) muscle fiber cross-sectional areas. Muscle total creatine concentrations were unchanged in placebo subjects. Muscle creatine was significantly elevated after 1 wk in creatine subjects (22%), and values remained significantly greater than placebo subjects after 12 wk. Average volume lifted in the bench press during training was significantly greater in creatine subjects during weeks 5-8. No negative side effects to the supplementation were reported.

Conclusion: Creatine supplementation enhanced fat-free mass, physical performance, and muscle morphology in response to heavy resistance training, presumably mediated via higher quality training sessions.