This study explores whether middle-school students can exploit explicitly addressed crosslinguistic lexical similarities between German and English to learn vocabulary more efficiently. Across six weeks, 260 Swiss German learners of English as a foreign language (17 classes) completed three vocabulary learning tests (T1, T2 and T3). Additionally, 7 of these 17 classes attended a 90-minute intervention between the first and second test: During a 45-minute introductory lesson students discovered four systematic orthographic correspondence rules (e.g. <p> to <f> as in "ship" and "Schiff"), followed by three 15-minute sessions to consolidate their knowledge. The intervention group's improvement in cognate learning was significantly larger in comparison to the control group. The difference was observed across both test modes and all rules introduced. While learners' initial English vocabulary size correlated with their overall performance, it did not affect intervention uptake. This suggests that these younger learners benefitted from form-focused instruction, independent of proficiency and type of correspondence.