BACKGROUND: To investigate how many rookie pitchers suffered from injuries while playing in the amateur league without guidelines for prevention of excessive pitching, we analyzed their amateur pitching patterns based on the pitch count, use of breaking balls, and pitches during winter camp. METHODS: Forty-one rookie pitchers who graduated from high school or university in 2013 and joined professional baseball teams. Participants were interviewed by a trainer using our questionnaire. Injury inclusion criteria were 1) history of shoulder surgery, 2) history of elbow surgery, 3) shoulder pain requiring treatment, and 4) elbow pain requiring treatment. RESULTS: Mean number of pitches per game and warm-up pitches for practice was 84.5 pitches (range, 15 to 130 pitches) and 16.4 pitches (range, 2 to 210 pitches), respectively. Mean number of pitches during the last year was 906.9 (range, 80 to 2,000). Mean number of maximal pitches was 127 pitches (range, 50 to 210 pitches). Fourteen pitchers had pitched over 150 pitches. Twenty-seven pitchers (65.9%) had pitched in spite of enduring pain. During winter training (mean 1.8 months), mean number of pitches per day was 162.5 pitches, and 20 pitchers (48.8%) had practiced pitching excessively despite the cold weather. Twenty-six rookies (63.4%) had shoulder pain or history of shoulder surgery, and 31 pitchers (75.6%) had elbow pain or history of elbow surgery. Only four participants (9.8%) did not have pain and history of surgery. CONCLUSIONS: For young baseball pitchers, guidelines for prevention of excessive pitching and for regulating the winter training program may be needed.