This paper shows that firms' demand for high-skilled foreign workers partly results from their hiring difficulties. Relying on a within-firm identification strategy, I compare recruitment decisions made by a given employer for similar jobs differing in recruitment difficulties. I quantify how the time to fill a vacancy affects the employer's probability to look for recruiting a foreign worker. To identify this relationship, I have collected and assembled a new and original dataset at the job level. It matches online job postings to administrative data on H-1B visas applications in the US. I find that a standard deviation increase in job posting duration increases employers' probability to look for a foreign worker by 1.5 percentage points. This effect is mainly driven by firms sending only a few visa applications. It increases to 3 to 4 percentage points for architects, engineers and computer scientists.