Summer vacations interrupt the rhythm of learning and may result in a loss of knowledge and skills. This study investigates summer learning losses in an Austrian sample with nine-week summer vacations. The results show losses as well as gains for students in lower secondary education (182 students between 10 and 12 years old). Students experienced losses in arithmetic problem solving (measured by the HAWIK IV intelligence test) and spelling (measured by the standardized spelling test HSP 5-9), but gains in reading (measured by the Salzburg Reading-Screening, SLS-8). Losses or gains in a knowledge domain appear to depend on the degree of practice during the summer vacation. Contrary to American studies, students could make up for their losses within nine weeks following the re-start of school. In addition, socio-economic variables such as the mother's educational background had a small impact on summer learning losses in arithmetic problem solving.