OBJECTIVE: To analyse time trends in overweight and obesity from 1991 to 2000 in samples of German children and to test the hypothesis of a trend difference between the samples from East and West Germany during this time period. DESIGN: Repeated cross-sectional studies using data of 35,434 five to seven-year-old children from school entry examinations in several rural and urban areas in East and West Germany (between 1991 and 2000). The main outcome measures were overweight and obesity. Weight and height were measured and body mass index was calculated. International cut-off points were used to classify overweight and obesity. RESULTS: From 1991 to 2000, the prevalence of overweight increased from 10.0% to 17.5% in the East and from 14.8% to 22.2% in the West. The prevalence of obesity increased from 2.1% to 5.7% in the East and from 3.6% to 7.6% in the West. All increases were significant. There was no evidence of a trend difference between the East and the West German samples. CONCLUSIONS: Unlike in other countries in transition, prevalences of childhood overweight and obesity were increasing in samples of East German children after reunification in 1990, possibly as a result of the rapid adoption of a western lifestyle in the East. Although prevalences were generally higher in the West German samples, there was no evidence that the increase was levelling off in the West. Overall, trends were similar in the East and West German samples.