This study explored the relation between second-language (L2) readers' memory for causal relations and their learning outcomes from expository text. Japanese students of English as a foreign language (EFL) with high and low L2 reading proficiency read an expository text. They completed a causal question and a problem-solving test as measures of memory for causal relations and learning from the text, respectively. It was found that memory for causal relations contributed to text learning in high-proficiency readers, but not in low-proficiency readers. The quantitative and qualitative analysis of causal question answers revealed that low-proficiency readers recalled fewer causal relations and made more incorrect inferences than high-proficiency ones. Additionally, low proficiency readers tended to perform the problem solving using inappropriate causal sequences and irrelevant information. These findings suggest that low-proficiency readers struggled with processes at both text base and situation-model levels; consequently, they failed to learn causal relations in the text as knowledge.