The short answer here is... No! There's a big debate in the fitness world on whether or not cardio will ruin your gains. The argument is that if you do more cardio all of the hard work building muscle tissue will be for nothing. Muscle is hard work and you must continue to work hard if you want to keep it and keep progressing. Use it or lose it as the saying goes! Cardio will have a positive impact on your gains. Cardio is short for cardiovascular which is your heart. Having a strong and fit heart is a good thing and will help with the efficiency in which blood and oxygen and other nutrients are pumped around the body. If you are a seasoned weight lifter, you will know of "the Pump". It's a natural byproduct of lifting weights. The muscle that is generally being used is filled with blood. The pump is a good thing and many people train in the pursuit of achieving a massive pump Having a strong heart will improve the efficiency of the blood being pumped around the body. This helps with recovery and helps you progress over time. Instead of worrying if cardio will ruin your gains, hone in on your nutrition! Adding cardio into your routine will increase your overall calorie expenditure, meaning if you are looking to gain or maintain weight you will need to increase your calorie intake. You might also need more protein depending on the situation. Adding any sort of EXTRA activity into your exercise routine will increase your calorie expenditure, this is what you need to watch out for. If you do more, then eat more, if you do less, then eat less.